Get dual monitor system and probably use a music player on one monitor and the game on the other monitor.
This way when there is dialogue you can easily just switch to the second monitor and turn off the music.
Admittedly not many people can be bothered getting a dual monitor display (I don't even have one) but for this type of thing it is just perfect.
Since SR3 uses .ogg you need to get the file converted from the newer .mp3 format anyway and most of the converters are dodgy at best.
Short Answer: Pay up or Pack up
EDIT: Another solution to this problem is to let the game play from your monitor speakers, but use an external device (phone, tablet, laptop, etc) and plug that into some external speakers. Again this requires money but it would allow you to turn down the volume when dialogue is happening on screen but you can still have music without having to TAB out of the game and restart the music.
Short Edit Answer: Pay up or Pack up... again