On android an empty apk in Godot is about 20MB. Installed it should be about 80mb. Then you need to add assets site. When you work with pixel sprites, you dont need to worry about the size.
For your audio files you should convert your MP3 and WAV file to OGG files to save memory.
I dont think there is a specific number for your question. I would say if you create something like retro platformer:
godot : ~20mb
sounds: ~ 1-5mb
sprites: ~ 1-4mb
Converting MP3 to Ogg is lossy (inflate to WAV, requantize, recompress…) and will not lead to significant smaller sizes.
Using Ogg or MP3 on uncompressed audio, sure.
Codecs (for coding and decoding) are algorithms that take a digital representation, requantize (re-calculate its digital representation) it in another format, losing some (superfluous) data, and then sometimes also compress it (JPEGs, etc) with another algorithm like zip.
Hope that helps,
Cheers !
Edit : inflate is taking the encoded or compressed data and expanding to its usable state