Setting up an office PA/music sytem via my PC

I would like to line out from the mini jack on my office PBX(which allows me to use an extension to make announcements over a PA system/stereo)& line-in to a PC. Then line out from the PC to a stereo(which acts as an amplifier)which in turn is connected to ceiling speakers. I know that this can be solved using a hardware solution, but I'm trying to use what I already have. So now I need software which acts as a virtual mixer so that it allows me to play music and as soon as I want to make an announcement via the PBX the music stops or fades out and the announcement takes priority. I can actually do this with the windows volume control but then I have to manually adjust the line-in & wave, volume sliders. If you have any thoughts then please let me know. Thanks in advance.