"Is it something to do with the fact that the incoming LP music is analogue?"...
Yes... that's exactly it!
Regarding the file format... the audio on an Audio CD is, by definition;
Stereo 44,100 sample rate 16 bit depth
Whatever softwrae you use to make/burn the audio CD will turn it into this... even if you "start" with, for example mono .mp3 files...
If HDD space isn't an issue you are probably best off creating the audio files as .wav files, with the specs I mentioned... it will make it easier for the subsequent burning software, and so reduce the likelyhood of burning errors.
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