The entity got renamed to snd_soundscape, but soundscapes are usable. Using them is the simplest solution, and they will handle starting and stopping the sound automatically. It will restart the music from the beginning if you leave one soundscape and come back to it though.

The other options involve using soundevents. One option is using trigger volumes do detect if a player is within the sound range, and using a snd_event_point to play/stop the sound. This would break down in multiplayer where you can have multiple players moving around. It's possible to play player-specific sounds, but you would need to use VScript instead, and the interface is still pretty limited.

Most of the options are documented here:
https://developer.valvesoftware.com/wiki/SteamVR/Environments/Adding_Sound

Thank you!