I'm also having issues with this. Audio will only play in reality composer and will not play in any exported .usdz or .reality files. I've tried starting the sound with Play Music, Play Ambience, Play Sound behaviors, etc. and none will play on any mobile device after exporting the project to a usdz or reality file.

Hi there, Were you ever able to figure out how to add audio playback to a usdz?

Hi there, I was in a similar scenario just recently and I found that I just had my iOS device set to "Do Not Disturb," which apparently mutes any sound coming from Safari's Quick Look player. Hope this might help anyone reading this in the future narrow down what the issue is.

Is your device on Do Not Disturb by any chance? My iPad Pro mutes audio from usdz or .reality files while Do Not Disturb is active.