Toroid Core Selection for Audio Isolation Transformer
A transformer doesn't pass DC. In fact, for a given design you will struggle to pass low frequencies through the transformer because, basically, the transformer primary is also an inductor that is in parallel with your drive signal. This inductor will overload your amplifier's output and make passing low frequencies a misery.

So, taking into account the core's ability to convert a given number of turns into a given inductance you calculate the number of turns needed to not overload your amplifier at the lowest frequency you are concerned with.

2 Hz is too low - for audio 20 Hz is fine but a lot of folk will decide on 40 Hz as the lowest point of the audio - you have to decide that. If you decide that at 40Hz you don't mind the impedance being as low as 100 ohms then this equates to an inductance of about 0.4 henries.

Look at the technical details of the core and decide how many turns are needed to achieve this inductance. Of note - doubling the turns quadruples the inductance.

Choosing the right size core depends on how much power you wish to pass thru it. You also need to choose a core that will work correctly (without attenuation) at 20kHz - most ferrites will easily achieve this but if using some other material then you'll need to look at the data sheet. It's probably a good idea to link the data sheets in your question.

Secondary winding is usually the same number of turns as the primary - this gives you a 1:1 transformer.