as we know the latest malware and virus (e.g. BadBIOS) hack your computers and steal your data by travelling through computer speakers, thus jumping over air-gap protection.
how do i protect my bitcoin wallet from these threats?
No, we don't know that. The BIOS dumps that were provided by the researcher claiming to have found BadBIOS have been analysed by lots of researchers and not a single one has been able to validate the existence of Ruiu's BadBIOS malware. Hardware and BIOS experts have weighed in, in addition to most of the heavyweights in security research. And...Nothing.
But, whatever, if you're really worried about air-gap jumping BIOS malware that traverses airgaps using sound, then there's an obvious solution: Don't attach a mic to the offline system.
What you should actually be worried about is malware that can hitch a ride on USB sticks, because those are actually known to exist.