Project Idea?

Looking for ideas regarding a project. I would like to build a circuit that allows 2 or more inputs to be combined, iphone and 2-way radio for example with priority channel override.

An audio mixer is an easy circuit. Just an op-amp configured as a summing amp.

For priority override, you would add a precision rectifier and a level detector feed from the priority channel, and use it's output to mute or attenuate the secondary channels.

Depending on how the radio is powered, if the iphone is to be charged from a mains charger at the same time its connected to this circuit, you might want to isolate the audio out from the phone, there is often a small capacitance from the mains to the o/p of a phone charger to prevent rf interference, this might upset a 2 way radio, esp lower frequency ones.
If you can find one an LT44 might do the job, or one of these car radio ground loop isolators.