What is the low frequency limit for ferrite cores in audio applications?
There is no low frequency limit - I'm running 6.0 Hz through a ferrite transformer in order to provide signal isolation to a circuit. I can set it all the up to 50 kHz too and there is no change in amplitude.

What I do have to be aware of is the lack of magnetization inductance that a hundred turns on an RM12 pot core offers. The overall low permeability of ferrites means I can't reliably use them as AC mains transformers at 50/60 Hz.

So running a signal thru at 6.0 Hz means I'm not pushing watts thru! 



EDIT SECTION

You have to be aware of saturation of the core. The RM12 (3C90 material) pot core that I wound had 100 turns but only had a primary inductance of 56mH. At (say) 50Hz, this is an impedance of about 18 ohms. With 3C90 material, looking at the BH curve for the material, it "starts" to saturate at a magnetization (H) of about ~50 ampere turns per metre: -



What does that mean you might ask! The "/m" part is the effective length of the ferrite core i.e. the distance the magnetism "travels" around the core and for an RM12 this is about 57mm so, the number of ampere turns you can apply is 50 * 0.057 = 2.85 ampere turns. As I have 100 turns, this means the pot core will start to saturate at a current of 28.5mApk.

The RMS will be about 20mA - given the impedance at 50 Hz is 18 ohms means a terminal voltage of only 0.36V RMS. At 1kHz, the impedance will be 20 times higher and therefore you can apply 7.2V RMS to the design before saturation starts to be seen.

So, ferrites have no low frequency limit providing you avoid saturation.