The only possible explanation is there a bug with your sound drivers which might have been updated without you realizing it after a Windows update. Since you say you DO have GSX sounds on the outside, it's clear the GSX sound is configured to the correct audio card and is working correctly.
The only thing that GSX does, when switching from outside to inside, is to set a specific OpenAL command to change the distance calculation used for sound attenuation, and letting OpenAL itself doing all the work. So, a bug in that function (the OpenAL implemented in your audio driver) might cause this, but it would be the first time that everybody reported it.