It's an invention. There's no real proof of music being played from mounted speakers during an assault that is supposed to be as big as shown in Apocalypse Now. There's very little truth to the way things are depicted in the movie (an argument can be made whether or not the movie actually tries to be historically accurate), but it certainly makes for an evocative scene. After all, much of the music and imagery from Vietnam War movies have been emulated over and over again so as to be intimately associated with the Vietnam War. It's an interesting thought about the power of popular culture in shaping the understanding and memory of a historical event to the point where the historical Vietnam War and the fictional Vietnam War merges together into one image.

Although what is depicted in Apocalypse Now did not happen in real life, music and other sorts of psychological warfare was officially sanctioned to both intimidate the enemy and to make them defect to the side of the South Vietnamese government.

The most common broadcast would be to use pre-recorded and/or live voices of VC or PAVN defectors telling their former comrades to defect and how good life was on the side of the South Vietnamese government. It might not have been that dramatic in comparison to 'Ride of the Valkyries', but it certainly had an effect and there were certainly some insurgents or PAVN regulars who did defect (120,000 VC and 2000 PAVN defected to the South Vietnamese side throughout the major part of the war).

However, there were some times when a dramatic display had to be put on and the following example, drawn from an event that happened in the autumn of 1970, showcases all the sound sensations that the 6th PSYOPS battalion could offer.

Let me start with setting the scene.

B Company, 2nd Battalion, 22th Infantry Regiment, 25th Infantry Division was out on a routine patrol when they discovered the entrance of a bunker with a note attached to it. The company Kit Carson Scout (a defected VC/PAVN soldier used as a guide/interpreter) translated the note which turned out to be an appeal of a VC unit in very bad shape, desperately begging local sympathizers for food, water and medical supplies. With this in hand, they turned to the 6th PSYOPS battalion which promptly put together a broadcast for the beleaguered VC to get them to surrender. Broadcasting from B Company's night defensive positions, the broadcast consisted of the following (beyond the usual pre-recorded defector tapes):

Wandering Souls. This tape, a recording of funeral music, spooky sound effects and voices, were meant to represent dead VC/PAVN soldiers and appeal to those who were afraid to die an anonymous death. In Vietnamese culture, the improper burial of an individual would lead to the individual's soul wandering the earth for the rest of eternity. One of the advantages of becoming a Kit Carson Scout was that if you died in action then your body would be shipped home to your family for a proper burial. That wasn't the case if you were a VC. You can listen to a recording of the tape through this link. It's a bit unsettling, but imagine that you're a VC guerilla who hasn't had proper food or water for a day, perhaps even longer and you're sitting in the dark hearing that tape being played on loudspeakers nearby.

Little NVA Sister / Crying Baby. This tape, which consists of a little girl pleading desperately for her soldier brother to come home. This was meant to target those young men who had left their parents, their siblings and their home to join the revolutionary cause. This could be effective considering how important on and hieu were. Since an early age, children were instilled with the custom and tradition known as on and hieu. They were taught that they owed their parents a moral debt (on) of such great proportions that it could never be fully repaid. So the children were told and expected to constantly try and please their parents and obey them. This was supposed to make them feel better for themselves, having reduced the burden of work on his parents. Anyone who did not follow through with this were rejected. Social standing was so important that it was considered that each person had to do this and try his/her best not to ruin the family position within the village. The hieu on the other hand was all about honouring, obeying and respecting his/her parents. They were always supposed to put their family's and parents' needs, expectations and wishes first before anything else. This included caring for them. The eldest sons also had an extra responsibility to take care of the family graves.

If that wasn't enough, the end of the broadcast would end with a literal bang. Artillery and mortar fire would be coordinated to hit suspected enemy positions and help fasten the process of demoralization.

This "show" would be repeated over several nights. In the divisional newspaper of the 25th Infantry Division, it stated that despite going on for three nights in a row, they had yet to receive any defectors or prisoners.

Followup question: The same music was apparently played when one US Army unit moved into Iraq in the Gulf War. Does anyone have more information?