What would the best way to filter high frequency noise? Decoupling/Bypass Capacitor? 1:1 Audio Transformer?
I do not see how adding caps to the 24 V line would help. The noise appears to be entering through the audio connection from the PC. Decoupling the 24 V for the monitors seems pointless to me. 

Regarding Audio isolation transformer: this could help if grounding is a problem. But if the noise is already present in the audio signal, the isolation transformer will not help.

But let me suggest a different solution for you which I use myself in combination with a high-end Headphone in order to prevent all audio issues.

I use a separate audio DAC, you could use this model, (mine is a different one with a headphone amplifier) and connect it optically (with a TosLink cable) to your PC. That is assuming your PC has an optical output. If your PC has a digital SPDIF COAX output, you could also use that.

If you do not have optical or COAX digital outputs on your PC you could get a USB soundcard like this one, this is the actual model I use.

I use the optical connection because it gets rid of any grounding issues. Also the audio signal does not become analog before it is outside the PC.