Mains transformer as line matching audio transformer
For music if you de-rate for the lower frequencies present in audio it should ok.  Strip-wound toroidial power transformwers have good performance across most of the audio frequency range.

If this is for voice only using a 120V power transformer on a 100V audio distribution line should work fine.

strip-wound cores have only one gap in the core, and it has a very large surface area, (being the surface area of the strip itself) E-I tranformers have a much smaller gap area, so the gap is more significant.  for this reason strip-wound toroidial and R-Core transformers are to be preferred.

Eddy currents are proportional to voltage, so you should be fine.

Saturation is proportional to voltage/frequency, so if you need to send 25Hz on a 100V line you'll want a 200V or higher power transformer.