To answer your question in part, I have music going while writing this post from my iPhone.
If you get a call while music is playing the music quickly fades out and the phone ringtone comes on & you can answer or ignore/dismiss the call. When the call ends your music resumes.
You can access apps while on a call, just hit the home button. A green bar comes up above the home screen saying 'tap here to return to the phone menu' or something like that.
One thing you can't do is switch between a chat app & facebook app & back to the chat without relogging in to the chat.
I hope this helps you.