Ground loop Isolation transformer, alternative?
If you use mains transformers for the audio signal it will 'work' but the audio performance will most likely be miserable. Perhaps suitable for voice. The reason is that transformers designed to pass many kHz need relatively thin and expensive laminations or the eddy currents in the laminations will attenuate the higher frequencies. 

You might be better off trying to salvage something from a very old transistor radio (not the output transformer but an interstage coupling transformer). Suitable line in/out transformers are sold for auto use.. not too expensive. 

Or get rid of the ground loop via a mains isolation transformer (large and not cheap). 

Or design a differential to single ended amplifier to deal with the common mode voltage.