I'm having trouble locating the specifications for the audio output. Where is this information located and what are the electrical specifications for the output?
Audio outputs: TRS connector | 3.5 mm jack, HDMI
S6: Audio connector: 3.5mm stereo jack (output only)
I also found this forum post:
The audio output is 1v peak-to-peak maximum. It's too low-level for headphones (according to reports - I've not tried)
Professional line-level 0dB is -or was, when first thought of - 1mW into 600Ω, which means 0.775v RMS or 1.09v peak, 2.18v p-p But you then need some headroom, and setting zero-level to 18dB below digital maximum means the kit should handle 10v p-p
What line-level means in domestic equipment is somewhat variable. Also, many (all I've come across) cassette recorders have automatic gain control (AGC), to handle that variability.
So call it line-level, and try it. You can play with the level somewhat on the Pi, to prevent clipping if it's too loud.
You'll probably be disappointed with the result. The 3.5mm audio out is approx. 11-bit quality, which is probably better than the cassette recorder is capable of. But also, the AGC screws up the quality, adding noise to the quiet bits and clipping sudden peaks.