By "default" sounds (mp3,ogg,wav) are placed inside "VAMfolder/Custom/Sounds/..." (you can organize by subfolders:music,sfx,...)
First you need to load/add sound into your scene to be able to trigger/play it.
goto edit Mode > Main UI > Scene Audio > Load (look for your sound effect/song inside "Custom/Sounds/...")
once loaded then you can trigger it to play from AptSpeaker, AudioSource or RhythmAudioSource/RhythmForce (or HeadAudioSource).
example trigger (for AudioSource):
Receiver Atom: Audio Source
Receiver: Audio Source
Receiver Target: Play Now
Clip Type: URL
Clip Category: web
Clip: <your sound>
btw. i mostly use .ogg format (but you can use .mp3 or heavy .wav it's up to you)