Yes. The playPreset() command can be used to play a preset based on the ID listed in the presets attribute. This could be a radio station, Spotify playlist, etc. as you alluded to. 
The original poster asked where they could host MP3s for use with RBoy’s app and I assume it’s likely for using an audio file as some sort of notification.
To your point about Sonos and playTrack(), my understanding is SmartThings is using the new Audio Clip capabilities of Sonos which are designed for audio clips / notifications / TTS files.
That’s part of the reason I was trying to understand what the context of @dman97’s question including what device he was working with and what he was trying to do with the folder of MP3s. For example, playing an M3U playlist on something like Kodi would be a completely reasonable and supported approach. 