I'm far from a expert hardware engineer but hardware like old phones transmit sound with a range of frequency that's translated by some oscillating circuit. It is dead hardware that doesn't do anything except transmit current that makes a small speaker vibrate at a specific frequency which create sound. It doesn't really execute instructions like a processor would.
Nowadays, you have complex microphones which capture sound. There are complex controllers in the microphones probably which execute instructions but not CPU ones.
For example, on x86-64, a USB microphone is polled by an xHC. The xHC polls the microphone until it has data. The xHC doesn't execute CPU instructions but it can write in RAM directly. This is definitely a vulnerability but is the kind of attack that would require having a combined control of the computer and to be in range of the microphone AND a vulnerability in the hardware controller (the xHC). The xHC does write in data but at the demand of software. Since it is DMA, if the xHC writes in RAM at some position where it shouldn't then, it is an attack vector. I have a hard time to understand why some sound vs other would trigger this kind of attack. I also doubt that there is such vulnerability in USB controllers.
The biggest threat is probably software. Software is audited a lot but is known to have lots of bugs and vulnerabilities. Software could overwrite some part of itself by triggering DMA writes for example. That could drastically change the behaviour of software and be an attack vector.
Don't get me wrong though. The question is "Do modern computers prevent instructions executed by audio? If so how?". The answer is they don't have to because audio isn't executing instructions. There is a lot of hardware in between and even this hardware is DMA so it doesn't execute instructions. It writes in RAM directly so this is the vulnerability not that "audio executes instructions".