Group ride-certainly not smart to do IMHO, there is too much going on that needs attention, not to mention that group rides, at least for the first 10-20 minutes, are time for socializing with the other riders in the club/team.

Solo ride- I have been putting one earphone in, in my right ear (away from cars). I have not had any trouble hearing vehicles approaching and the music is nice. Only problem is when songs that are not in stereo come on, you only get one channel and they sound pretty funny. Try listening to the Beatles or older Stones stuff in one ear phone.