[MetalDave]

When you add new tracks the db level will go up as you add more things to the mix you will need to turn other things down. This is perfectly normal. If they are kistening to a complete album, they volume of the last song should have them sitting at the volume they want. the next song starts low and they will usually wait until all the music comes in before thinking about turning it up. If you want it to be loud you can also envelope the track and make the single thing louder, then turn it down into the mix when the rest of the music starts.