Standard Windows Media Audio from what I can tell.
However, one unique thing about PGMusic products is that whatever the system itself is set up to use can come into play. If you have Professional, it will likely use it (once the sound is on/in your computer). but for the included audio files I think they use regular WMA so they play on all systems.

/this is not an authoritative PGMusic endorsed answer ..

I do not work here, but the benefits are still awesome
Make your sound your own!