You don't need audio panel as mentioned. Most Com radios and intercoms accept Aux audio input.
In my case I have a dedicated intercom with Aux inputs. Control of individual source volume, what I hear is at audio source (volume control). I hsve overall volume control on intercom. My Com radio has standby freq monitor, so it's almost like having two radios. I don't need to listen to GPS. Music is the other input.
On my first RV I made my own mixing panel, simple audio mixer. Again most Com radio's today have intercoms to handle Aux input(s) easily. I went with Stereo Intercom. I have to say a small piston plane is not a place to listen to hi-fi stereo.
In the old days your audio panel had the marker beacon (MB) lights and audio, and 4 radios ( 2 Coms, 2 Navs - to ID or listen to FSS). You could select what to listen to and what transceiver to transmit.
Related but slightly off topic, Many new GPS ( fancy Garmin IFR type GNS 650/750) have the audio panel built in, with touch screen control and graphical depection of who hears what (multi place intercom).