Electronic – Whether to share bypass capacitors

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.

Let me preface this with: decoupling is generally something you do as a rough judgement call based on theory principles; I haven't ever done it by analysis at a board level (versus IC level, where I have done back-of-the-envelope time-constant analysis). If you later find that you used too many caps (brings cost up) or not enough/not the right values (performance problems), then you'll adjust the caps experimentally—it's good to leave extra pads on PCBs for additional parallel caps you think you might need if things go wrong, at least on prototypes, sometimes even on a final PCB.

To answer your questions:

1. It depends on transient currents and distance (or more to the point, trace inductances). For 10μF particularly but 1μF too, you can probably get away with a few mm or low cm of traces and use only a single capacitor, and I would probably go with only one on a prototype if the final space/BOM cost is an issue. Upgrade to the two caps if you find one only one causes issues, or if space permits leave some pads for two caps closer to the pins. The 0.1μF will be much more sensitive to distance so keep that one closer to the chip's actual pins.

2. Depends on distances and the amount of transient currents the cap has to filter. If your voltage regulator output is right beside this chip (and the transient currents aren't terrible—if they are maybe consider increasing the capacitance), you can just use one 10μF for the entire thing. If your voltage regulator is a handful of centimetres away, you'd want to put in a 1μF or 10μF closer to your STP24DP05, along with the one near the voltage regulator.

3. Excessive number of capacitors, no, except it costs more and takes more space! However, trying to put big capacitors instead of smaller ones can cause issues—large capacitance values help filter bigger transient currents which is good, but physically larger capacitors cannot handle fast current transients as well. Capacitors are less effective as frequency goes up, and physically big capacitors can't go as high in frequency due to higher equivalent series inductance; see this other question. This is why we put parallel capacitors of different values in the first place.

Someone once told me that, as a general rule, using parallel capacitors two decades (factor of 100) apart provides good coverage of frequency range. So 10μF and 0.1μF, or 1μF and 0.01μF.