You say when you “add” the audio, you mean when you create a Multicam with just the audio, correct? Adding audio to an existing Multicam clip doesn’t change anything. But you can override the defaults.
Select all the clips to include in your Multicam.
Right-click, select “New Multicam Clip”.
Click the bottom left button, “Use Custom Settings”.
Set it up as you need it to be, it will ignore the media’s specs at that point.
If you need to change the frame rate after the fact, can’t be done on a Mutlicam clip. Don’t ask me why, but Apple decided it can’t be done at that point. You’ll have to create the Mutlicam clip from scratch.
When using audio only clips, FCPX uses the sample rate of the audio. But audio has no frame size nor frame rate, so FCP defaults to TV broadcast standard. Ideally you’d include all audio and video clips in the initial creation of the Multicam Clip. But if you have to start with just the audio clips, again, there’s the “Use Custom Settings” button at the bottom left which lets you override to your own specs.