You do not need any to play Syrinscape content yourself. But if you want to trigger music or sounds from in Fantasy Grounds you will need the Audio Overseer or Mattekure’s extension. I do not know anything about Audio Overseer, but I’ve had extremely good times with Mattekure’s. John is very helpful on the forums here and on Rob2E’s discord.
The FG extensions are not required at all. They provide a convenient way to trigger the sounds from within FG, but you can certainly use the Syrisncape Online Master Interface to manually start playback. The extensions do not handle sending audio to your players at all, which is why the Online player is so nice. It handles all of that for you.
For any that are not aware, an FG Unity update last week massively sped up the loading of large lists like the syrinscape module. My previous loading times were anywhere from 5-10 mins when loading the full 27,000+ entries available from a supersyrin subscription. After the update, it takes about 4 seconds.