Hi,
If used in signal path then a capacitor indeed needs to be dedicated for audio use, otherwise you should only care about ripple and noise suppression, so I really don't think it matters much if Elna Silmic or Panasonic FM if used for decoupling (I would care more about the no. of hours that cap was designed to be used for). Basically, a very good low-noise power source is a must in Hi-Fi world, but for opamps decoupling a small and very fast ceramic capacitor of 0.1uF (for fast transients) and a low-ESR 10uF tantalum or 100uF electrolytic should be more than enough for decoupling (http://www.analog.com/media/en/training-seminars/tutorials/MT-101.pdf, http://www.partsconnexion.com/PDF/mulcap-ps.pdf, http://www.capacitorguide.com/coupling-and-decoupling/).
If sound is indeed changing when switching between different decoupling caps then using a scope might see this difference, so either the opamp is or is not oscillating anymore, either the ripple and noise on opamp V+/V- is indeed different (depending on the caps used, one specific cap might perform better than another under some circumstances).
When used in signal path, then "best cap is no cap", so DC servo and a simple "wire" would be better, if possible. Otherwise, we all need to stick to the best audio cap we can purchase, that means very low:
dissipation factor,
dielectric absorption, low inductance,
ESR etc. ()