To answer your second question first, in large crowds some cultures would station people to relay the message they heard.

So you would have some one speaking and then maybe 5 or 10 people so far out shouting the message as they heard it.

As far as other armies are concerned I'm not sure, but the Greeks apparently did this quite frequently. An army commander would stand and deliver a motivational speech to the army before joining the men in the ranks.

As armies got larger, or more importantly had force multipliers that allowed them to cover more ground, this probably became impossible.

It's a movie trope for good reason, it's a bit of a military tradition for the higher brass to intermix/speak with the lower ranks before a battle. O-7 and up to general grade officers have a bit of a celebrity status in the Army. We may b**** about them constantly, (usually with d*** good reason) but we'll still crowd around if they pay us a visit. It's kind of like how celebrities are treated in the civilian world.

More importantly, they can serve a critical purpose. Great officers MUST first be great administrators, secondly great tacticians, and finally good motivators. Horrific morale disasters have often been averted by a single officer's charisma.

When General Washington gave his "Farewell Address" to the Continental Army, they were on the verge of mutiny for not getting paid by Congress. He calmed the situation and basically saved the shaky government.

Though I don't remember a single speech that served as a catalyst, Washington's actions at Valley Forge were incredible. He managed to hold his army in the face of defeat, terrible weather, and worst of all, woefully inadequate logistical support.

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Here are some of my personal favorite examples from Generals motivating their troops throughout history....

General Robert E Lee after the Battle of Spotsylvania: [full transcript]

“Some of our bravest officers and men have fallen, but their comrades not less brave will emulate their glorious examples.”

General Eisenhower's Order of the Day speech before the pivotal invasion of Normandy. (Audio included)

"The tide has turned. The free men of the world are marching together to victory. I have full confidence in your courage, devotion to duty, and skill in battle. We will accept nothing less than full victory."

General Hannibal Barca: (Not a speech I know, but still incredible.)

‘It is astonishing to see so great a number of men.’

Hannibal sensed his anxiety and decided to turn it his own way:

‘Yes, Gisgo, you are right, but there is one thing you have not noticed.’

‘What is that, sir?’ asked the puzzled officer.

‘In all that great number of men opposite us there is not a single one named Gisgo.’

General 'Stonewall' Jackson before the First Battle for Bull Run: (movie scene)

In the Army of the Shenandoah you were 1st Brigade!

In the Army of the Potomac you were the 1st Brigade!

In the 2nd Corps of this Army you ARE the 1st Brigade!

You ARE THE FIRST BRIGADE in the affections of your General.

And I hope by your future deeds and bearing, you will be handed down to posterity as the 1st Brigade, in this, our Second War of Independence!

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Speaking from personal experience...

The two biggest speech makers I've known were my PG at Basic (Fort Benning,) and my old First Sergeant. Neither of these are officers but I think you'll get the gist of what a good pep talk can do.

The former was a VERY powerful and charismatic individual with an 18X contract. We got into the tradition of holding an unofficial AAR at the end of the day (post Red Phase); after that, he'd give us a little pep talk to keep our heads in the game. On our last night before graduation he gave our platoon and me the most motivating ten minutes of my life. (We recorded it on a cellphone, I'll try to find it.) He ended his last speech with a quote I've since written on the soles of every pair of boots I own:

"Once more into the fray, into the last good fight I'll ever know. Live and die on this day, live and die on this day."

After he said that, our whole platoon exploded in a celebration that lasted most of the night, screaming "HOOOOAAAHHH!!!!" Thumping lockers, the ceiling, and each other.

My old First Sergeant had a different approach....

"I've been shot twice, stabbed once, and blown up I don't f****** know how many times. I've done my part and want to go f****** fishing. But if I hear about y'all f****** up, don't you think for one second that I won't drive 200 f****** miles to personally mushroom stamp y'all on the f****** forehead."

I actually posted a list of his best quotes on /r/Military a while ago as a tribute to him after he retired in July. You can check them out if you like. (Lots of language though.)

Hope this helps!

EDIT: Broken link, profanity

EDIT: More profanity

The pre-battle speech was also a staple of classical histories. u/TheChivalrousRogue gives an example of a speech attributed to Hannibal, the speech attributed by Tacitus to the Caledonian leader Calgacus (chapters 30-31) is also pretty well known:

We, the last men on earth, the last of the free, have been shielded till to-day by the very remoteness and the seclusion for which we are famed... But to-day the boundary of Britain is exposed; beyond us lies no nation, nothing but waves and rocks and the Romans, more deadly still than they, for you find in them an arrogance which no reasonable submission can elude... Robbery, butchery, rapine, the liars call Empire; they create a desolation and call it peace.

However, this is almost certainly an original composition of Tacitus'. He says vaguely that 'we are told' that Calgacus used 'words like these' but it's hard to see how Tacitus would have known this.

Thucydides even explicitly says that while he has tried very hard to ensure that the events he recounts are accurate, the speeches will mostly be what he feels were required by the situation. Thucydides is quite a careful historian for his era, and lived very close to the events he wrote about, so it's probably a lot worse for someone like Plutarch.

This convention was also fairly common in the middle ages, of the material I remember, or have to hand, Marco Polo has the generals of two opposing armies give speeches before battle in chapters 52 and 53 of his 'travels' (sorry don't know quite who they are, the generals are named 'acomat' (ahmed) and 'argon' and they're from 'great Turkey' wherever that is). The chroniclers also give Henry V a fairly involved speech before the battle of Agincourt (Shakespeare replaces this with the 'band of brothers' speech, so there should be a fair bit about the original on the internet).

As the other posters have said, real speeches were pretty common before battles, but as far as I know, most classicists believe that ancient historians weren't too interested in what they actually were.