If I understand your problem, here's what I do. (Of course there are limitations). You can make your gif in IR and load it on your site and then embed a wav file as back ground music. Set music and animation to autostart or whatever.
Flash does support audio and you can even edit there too. Even the Flash 4 supports audio. The advantage of using Flash is that you can coordinate the music with the animation. I have hear Swish is easier to use. Flash can be a problem unless you have a well written book by an expert. I like the "beginning-intermediate" books at peachpit.
The way I suggest, you can use IR for animation and enjoy sound too. Wav files play on almost all computers. If it is embedded as background, it streams so you don't have to wait for it to load. Well modems may take a bit longer.
Good Luck let us see it when you finish.
Joy [excited] [excited] [excited]