I use a bench audio amp and a audio select / conbiner panel. The conbined Rx audio is muted as part of the transmit sequence. Once you get beyond plugging the mic into a couple radios, things start getting fairly complex, with muting, and speaker audios etc. Don't know how/if you are going to switch your mic audio, but I put together a 6 station controller for distributing the mic audio, and Key line, along with a universal mute line, with minimal/reversable mute cktry in each receiver. The basic mic audio distribution is a pretty simple concept, but it, along with the Key line, and receiver mute functions, that can interface any and all comers requires a few switching relays and a fair amount of cabling. Unlike you, I only have 6 "rigs" on the bench at a time.