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First, thanks for all the links, they were very useful. Second, I'm sorry to say I still didn't find a solution for us. After reading some material beyond the links you posted it really seems that Unity is not handling very well the async processes that use the audio stream on Android.
Worst, it seems a bit subjective if it's an Unity problem itself or a [specific Android version] x [specific Unity version] issue, the worst possible scenario to try to detect and solve.
I've tried some almost-random stuff, only because I could, like using AudioHog and Audio Refocus, tried a lot of tests but no luck so far =/
Well, so far our only hope is that #1. Unity get their sh*t together and #2. Apps update themselves with the new shitty Unity, probably fixed..
Anyways, if I find anything else I'll keep you posted.