JonnyBNB wrote:
My brother in law and I were tuning a new antenna for his Cobra 18 WX
ST and when we got the SWR as low as we could, tried it out. As he got
further away during a test run I encouraged him to speak up because he
wasn't getting decent modulation. When he did the result was pulsating
oscillations that obscured his audio.

1:1 SWR does not mean resonance.
It just means you've made the antenna about 50 ohms. You are probably
getting RF down the shield of the coax and causing RF feedback. You
need to decouple the antenna from the shield with a proper ground plane.

Are you sure you didn't pull the ground loose from the base of the antenna
or the crimped on PL-259? If this is a mobile installation on a trunk lip,
don't trust the hinges to have a good RF ground. I guess this is a magmount?

Since you didn't describe your antenna, we can't help any more then this.

I'm an audio guy, but I mix rock concerts. The type of squealing I
usually deal with is some jerk guitar player wanting 130 dB SPL from
his monitor wedges. There's not any possibility for audio feedback in
this case, my rig was two miles away from his and he doesn't have
talkback.

And I have hundreds of GB of lossless music and am a shn/circ manager
at db.etree.org and also a moderator of a major BT site. When a famous
old band played locally, the main man made sure I was on the guest list!

At any rate, the question is, would replacing the audio amp chip do the
trick, is the part readily available, and is it anything a guy with
plenty of soldering experience couldn't tackle.

No, replacing the amp chip won't help. Fixing the antenna's problem will.

Try your local library for an ARRL handbook. It should be somewhere around 621.