Elfa,

I want to match the impedances by making my own small audio transformer

I'll go out on a limb and say that a home-made transformer will probably
have more losses (and other problems) than any efficiency gain you could
possibly hope for. This is not to discourage you from experimenting! Go
ahead and make the transformer anyway. But don't be disappointed if it
doesn't work well.

You mentioned having a toroid ring. Audio transformers need to work over a
wide range of frequencies. Is your toroid meant for audio, 60 Hz power, or
radio frequencies?

--Ethan