Not sure that I have quite grasped the implication here. Is it in the region of - 'that's all I can hear', - 'I can't hear anything else' ?
No. That would be חוץ מזה אני לא שומע כלום. The sentence here means "otherwise, if I don't do this or if that doesn't happen, I can't hear anything". It's assuming some context and previous knowledge. אחרת always refers to something that was said before.
Because everything(הכל) is different from anything(שום דבר, כל דבר). To my knowledge, שום דבר/שום I only used in a negative sense; "I cant hear anything" rather than "Pick anything you like".
That translation makes sense, but it might have been critical about it since "כלום" literally translates to "nothing".
You probably made a spelling error, or it was a Duolingo bug. (Please don't use caps, it's considered shouting). Also, as a side note, it's "accepted" not excepted.) 27 February 2019