John:

Do you know what % that generally used for slowing down footage in videos?

Quite often 2/3 - so if you are shooting for 25fps (which I do) then it wouldn't be unusual to sync your sound to 37fps. But realistically you could sync to anywhere between 30fps - 37fps for 25P depending on the look you are after. Try to timecompress the audio so you don't pitch shift it (ie just speeding it up) - easier for the talent and if you can get a timecoded slate with a timecoded CD/DAT as playback it can help a lot in post. That way, wherever you pick the song up in a take you have a consisten timecode point to work with. A playback operator with a prope rig can be worth the cost.