I am running XP as well. I was listening to music one time when I decided to stop and play GM. I have a start-up song (Norther’s “Final Countdown”), which started much, much louder then the music I was previously listening to. Try going to YouTube, watching a video at regular volume, and then without touching any volume controls, start GM. If my hunch is correct, GM will be much louder. So your system isn’t getting quieter, GM is just louder. Turn the volume in GMod down if it really bothers you.
Go into control panel, and look for “Sound” or some other sort of audio manager if you have a separate sound card. This varies per sound card and per OS, but just poke around until you find an option called “loudness equilisation”, then disable it.
There’s an option on Windows 7 that lowers sound settings when it detects communication. Right click on the speaker icon in you tray and select sounds. Then go to the communication tab, and select “Do nothing” Apply, and it should be fixed.