I've already done the same thing you did, but with a much cheaper set of headphones! First, I've only been riding for a little less than a year, and after reading posts from enough people about wishing they had worn ear protection right from the start, I won't ride without something in my ears.
Anyway, my experience with my "homemade" helmet speakers was the same as yours. You can wear earplugs while using helmet speakers; it actually cuts down wind noise, but requires more volume out of the speakers... it just didn't work out for me.
I got a reasonably priced pair of skull candy earbuds, and to my suprise, I was able to get my full face helmet on without them ripping out of my ears. I do have to pull them out before taking the helmet off though. They do not provide as much ear protection as the foam plugs I use, but this was many times better than the helmet speakers for listening to music at any speed. I would bet that if I was willing to spend more money on better earphones, I could find something that would provide adequate ear protection and decent music quality.
I wouldn't give up on the earphones yet unless you've already done alot of research and tried different pairs because I don't think helmet speakers are going to work out for you. Maybe if you get a huge wind screen to hide behind to eliminate all wind noise, helmet speakers would work.