Are we talking about saturation remanence or
coercivity?  The Hc of
 computer and audio tapes are approximately the same, but the Mr is
 quite a bit different. 
I'd assumed it was the coercivity that was differnt, but mayne it's the
remanence. I will admit I've not done any measurement :-).
I am pretty sure that audio cassetes (even with a notch cut in them) do
not work well in ECMA34 tape drives. And that the data tapes make lousy
audio tapes at any normal bias setting.
-tony