Silent films were still being produced in notable numbers until the 1930. Often due to the expense and novelty of the technology, not every film released in the early sound era could have sound scenes throughout. There were in fact dozens of films that were for the most part silent, but had an occasional sequence of sound. An example of this is the Paul Fejos film Lonesome (1928). Since many theaters didn’t have sound systems yet, studios would make two versions, a completely silent one and one with the sound scenes for the cinemas equipped with the technology to play it. Sometimes to cash upon the trend, a studio would add last minute sound scenes. Due to the abrupt addition of such segments they had little impact on the plot and were often cumbersomely inserted. The practice of adding sound scenes to already silent films was known as “goat-glanding”, after the bizarre pseudoscience trend of the 1920s in which men would have their testes removed and replaced with a goat’s. In re-releasing silent films made before the sound era back to theaters, some studios dubbed or even reshoot scenes, such as the 1930 re-release of The Phantom of the Opera (1925).

By 1930, virtually all theaters in the USA had the ability to play movies with sound, making the practice obsolete. According to Turner Classic Movies the last Hollywood film to have been made with both silent and sound edits was George Fitzmaurice’s 1930 comedy Raffles.

Several notable film directors however continued to make entirely silent films in the interest of creative expression, making the resistance of sound film’s growing domination of the market more of an artistic cause than a commercial one. Among these include Tod Browning, Luis Bunuel, Charlie Chaplin, Alfred Hitchcock, F. W. Murnau, and Yasujiro Ozu. However, by the mid-1930s this practice had died out, with the above directors transitioning to sound. While films that were homages to the silent era have been release since, featuring little to no dialogue or sound, the silent era of movie production in the USA had commercially ended by 1930 and artistically ended a few years later.

Film theaters underwent huge changes themselves during the transition to sound, and I figure I'll cover what that change consisted of. Here are some of those changes, but note we are talking about a period of only a couple years

1. The End of the Prologues

These were live acts, usually of local talent engaging in talent shows, sing alongs, vaudeville or other kind of live revues, that independent theater owners would use to attract audiences. Hollywood studios, not a fan at all of prologues (since revenue film producers could be making went to pay for these live acts) and they pushed sound films to help eliminate them. Filmed Broadway shows and comedians with national radio airplay, coupled with theater owners not happy with high fees and unionized orchestras, saw to it the demise of live attractions. Why settle for local talent on stage when you can see and hear a great Ziegfield folly on the screen? Theater owners responded to their audiences with the goods that Hollywood provided and prologues were soon out the door (and with that, some control theater owners had over what to show).

2. Sound Theaters

In the summer of 1929 over 25% of American theaters were wired for sound, mostly in the big cities. However, this trend of sound theaters was found in many midsized and small cities in the chains owned by the studios. Not too many in each area, but enough that you were in distance of a sound theater if you owned a car, so there was demand. This put pressure on the 75% of theaters (independent ones) that did not have sound, in areas that had a low population level or were not well off economically. While studios provided them films, they wanted their own chains to succeed, and the high production of sound films in 1929 put a lot of pressure on the small-timers. Audiences could just drive to a sound cinema, and many independent theaters sold themselves to a chain, closed or had to borrow money to get wiring (to have wiring, you practically have to tear up the theater, add chairs to absorb sound, silent ACs...). Hundreds of theaters were closing before the stock market crash, mind.

Small theater owners, sensing the change and upset over what they saw as unfair practice from Hollywood in producing mostly sound pictures they couldn't play (plus many other unfair deals with distribution, such as movies never delivered they paid for, and the majority of sound films being of shoddy value) banded together to help bring the studios to court, though that didn't work.

3. People stop going to the theaters

1930 saw a drop of people going to the theaters as their source of entertainment. Chains only made up 20% of theaters, and independent theater owners closed shop. Gimmicks like free movies after a showing, or handing out cigarettes or weekly gift piece nights were the norm, a losing one. Television was tried (with little success) in New York. Mickey Mouse clubs were one hit though.

One substantial change had to come in theaters themselves. Improving sound quality was a number one item for many theaters (sound was rather spotty at the time) so fixes to things like the volume helped improve audience numbers. Another change was the snacks people wanted - popcorn, much quieter to eat than peanuts, became the mainstay of movie-goers. Air conditioners, which had to be installed to help with sound movies (removing fans and closing doors), kept movie theaters open during the summer. Running double pictures (A- and B-pictures) also became part of regular programming. Some theaters though, unable to grab audience's attention, even introduced mini-golf inside the auditorium, removing seats and whatnot (it didn't work at all as mini-golf as a fad dried up). Nevertheless, many chains were hit hard by debt and you can imagine how small owners fared. (As a sidenote, sound films had come a long way by this point in 1930 since 1927....when The Jazz Singer was re-released in 1931 in an exclusive two-week showing, it flopped and was cancelled in three days - it was already seen as outdated by audiences)

4. The Depression does non-sound theaters in

As the 1930s went on, theater owners who could not or would not convert to sound theaters were eventually forced out of the business. As independent theater owners lost control over what they could show (such as getting rid of prologues in 1929) and saddled with unfair distribution practices from the studios, they saw their fortunes dry up as people chose to go to sound cinemas within driving distance of many Americans, or to other amusements at the time. People liked sound because it enhanced the movies - it was familiar (they weren't that different from silent films except in one big way) and audiences stuck to them.

This was taken from the fourth volume of the History of American Cinema, The Talkies: America's Transition to Sound 1926-1931 by Donald Crafton. I summarized many of the book's points, and hope this answer suffices (my first on this subreddit)