seems to me that you are asking if the LiveAudio plugin is already available. unless it has already been loaded i would think this would fail on most NS implementations. you probably have it loaded on your V4 and not on your 6. and i sure don't have it loaded on my NS7. the plugin may work fine in NS7 - i just don't have it.
i am sure i have seen a test for a plugin and if not there will download it (if the user accepts). not sure where i saw that example.
One of the frame pages will stay open to serve all the other frame pages. Usually that would be the contents frame, (The one with the menu.) So put your music in it. It should play until you leave the site.
Music can be a sudden shock to someone that doesn't expect it to be there. Especially if the volume is too high. It might be better to give a choice to turn it on/off.
I think embed is now compatible in both browsers, and is preferable to bgsound. If you use both, you should code embed first, or the sound may be distorted.
<EMBED SRC="startrk.mid" volume="50" LOOP="false" hidden="true" WIDTH="100" HEIGHT="40" autostart="true">