Been trying out a lot of different music pieces for missions, there's a scant few lyrical songs that ever came through my listening or play testing, it just distracts and changes the mood drastically. Only the ones like Immediate Music, used in WiH, but they have to be placed well. If I recall correctly, that music piece has been edited somewhat to fit better than the original piece, but I suppose that may go without saying if you're planning to add custom music.
For music preferences for FS2 I suppose anything that resembles the original score is great, but also powerful drums, though they can occasionally be too loud.
On the subject, is it possible to get into trouble for uploading missions with custom music that may be copyrighted in one form or another, even if credited? It's kinda holding me back from using some good sources myself and may be important to consider for anyone.