just lying back on my bed with some headphones, and breathing in through my nose and out through my mouth, while listening to some tranquil music?
If you sit down and truely relax yourself long enough, you should get into a deep trance eventually (if you don't fall asleep or loose your awareness).
Definitely--that's really all I do, except I sit up. I can't get into an altered state using any of the countdown, breathing methods I've learned so I do exactly what you described. You will probably have to give it about 25 - 40 minutes and keep in mind that, if you are trying to have mystical visions or journey, laying on your back, facing up, will put a different spin on things then laying on your stomach will. Sitting up is pretty nuetral, though. At least, this is what I've noticed. :-)
Once, when I was about 16 or 17, I was listening to some Jimi Hendrix instrumentals through headphones when I was lying in my bed in the dark. This was way before I knew anything about AP. As I had my eyes closed just listening, I started to get the feeling that I was floating and twisting and turning. It scared me, so I immediately got up and started to walk around the room. A few years later I learned what was going on.
So yes, a trance can be reached by just passively listening to music.
I have this track by Anugama called "Shamanic Dance" which is just a slow continuous chant in the background with a drum being pounded at an even pace and a rattle further into the song with some flute action too...I listened to it before I went to bed the other day and I was able to get into a decent trance by listening to it.
I think like most things in life the best way for you to get into a trance will be found out by experimenting because everyone is different.