Measurement current transformers have specific requirements, particularly high µr to reduce the error due to magnetizing current.
As far as reported, your application doesn't impose these specific requirements, it sounds like a regular audio transformer problem. Frequency range and transferred power are the relevant parameters for core selection, in case of doubt a regular transformer core would be a good choice. Nothing against a toroidal core in general, but it sounds like a way to make your life harder than necessary.