It's because when you start resizing window your main thread which pumps messages (at least on Windows) enters modal loop. It means that code execution stops during DefWindowProc call. That's why OpenAL consumes all available queued buffers and stops playing anything.
To solve this you have two options:
* when you are updating and queueing buffers, you also check if music is playing - if not, then resume playback (just issue play command). Of course user will notice silence when window is resizing, but he will get back his music.
* a little bit harder solution is to move your music update loop to another thread. This involves some basic synchronization between threads when communicating about which music must be started or stopped. But at least your music update loop will be executing even when user is resizing window.