This KVM has HDMI input and can directly emulate USB mass storage; fan-modulation is the lowest-bandwidth (side-)channel available to the attackers.
If the microphone was used for exfiltrating data, it would work against random targets that happened to let the KVM connect to the internet, and who have a nearby machine infected with some malware. That kind of non-targeted attack can be damaging but is semi-useless to the attacker.