Only problem there is that PC power supplies use switching, not linear regulators, so there will be high-frequency components possibly messing with things (hopefully inaudible, but…)
E.g. Doepfer’s current supplies use switched 15 V supplies followed by linear regulators to get around this.
(also PC supplies are designed for heavy load on 5 V and not so much on +12 V and even less on −12 V, but if it works it works…)
If you don’t mind switching, a cheaper/more compact alternative to a PC supply is a dedicated triple output supply, e.g. a suitable model from Meanwell’s RT-B series that philippejadin@ mentioned here. Very inexpensive, and a quality brand, but make sure you put it in a suitable enclosure.