

I think your fundamental problem is that you’re feeding the radio with a reversed positive voltage instead of a negative voltage, which fucks up the grounding. Your PSU setup is generating 0V, 5V and 7,6V, when it should be generating -7,6V, 0V and 5V.
You might be able to make it work that way, but it might also cause problems (such as humming) which could be near impossible to solve.




Yeah, that part is fine on its own. The positive terminal of the power supply becomes ground, it’s connected to the chassis of the radio, which will have roughly the same voltage as the earth itself. Switched power supplies are usually floating, so you can decide if + or - should be grounded.
The issue appears when you power the Pi from the same power supply. As you’ve discovered, what the Pi thinks of as ground then is actually -7.6V (compared to the voltage of the radio chassis/earth).
The audio signal isn’t actually reversed, it’s an AC signal and is more or less symmetric around ground. The problem is that your two devices don’t agree on what ground is, because they are powered weirdly.
You may be able to get away with keeping the PSU setup as it is, and just connecting a large capacitor in series with the ground connection. That’ll be way cheaper than a transformer.