I've got a Bose ae2 headset and the same problem: volume keys do work on ios but not on android, whatever your device is.
I've tried several smartphones (htc, Samsung, sony) and none of them handle those keys.
I've also tried Smart Keys but i don't see any option relevant to volume keys.
I quit trying to solve that because i think it's just hardware, on the headset side.
Maybe with the fact that Android devices greatly outnumber ios devices, sennheiser, bose and others will reconsider this problem on future models.
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