Extron
does have audio-only switchers, although I was only able to find
6- and
12-input models, each available with balanced Phoenix and unbalanced RCA variants.
There doesn't appear to be any of the RCA models on eBay, but there is a
12-input Phoenix variant for about $55 USD. Alternatively, you could pick up two of the 6-input ones and daisy-chain them for 11 inputs.
If you want to accommodate RCA audio without building your own adapters, you can either get 13 of these
dongle-style adapters, but it'll probably be cheaper to get four 20-position* 0.5mm Phoenix connectors on AliExpress (Whichever switch you buy probably won't include many, if any, of the Phoenix connectors) and 13 of these
RCA to screw terminal adapters.
* These switches appear to use 20-position receptacles, which means if you get a bunch of 5-position ones, they're likely going to have fitment issues; I had this problem with my CrossPoint, which uses 10-position receptacles, and the 5-position connectors had too much plastic on their sides to fit next to each other within the same receptacle.
Out of curiosity, how does an audio-only switcher fit into your setup, and what is the purpose of going audio-only versus an AV switcher (especially if you intend to be manually switching inputs)?