Lets say first of all, I am not a sound person. I am currently in post-production with a short film which has no dialogue, only sound effects and music. All the sound effects are done in post. The short is being showcased at a festival in the summer.
The sound effects are in stereo (a1 and a2) on the timeline.
The music is in stereo (a3 and a4) on the timeline.
I am exporting to prores 422 with audio stereo 2 channels 48khtz
What is the ideal sound codec and what pan do I use before I export?
What should go left and what should go right and so on?
The only easily answered question is what codec and it's uncompressed 48kHz (16 or 24 bit is up to you - delivery is going to be 16 bit and the internal processing is at 32bit so that doesn't make much difference. I'm sure there are some who would say that 24 bit is better and they'd be right IF you're delivering at 24 bit and your sources are 24 bit.)
When it comes to track layout, I'd use more tracks, say 1/2, 3/4 and 5/6 as effects tracks and 7/8 and 9/10 as music tracks, to allow for overlaps.
Panning is a matter of taste. Stereo music and effects tracks already have those decisions made, so the left side goes entirely to the left and right entirely to the right. With stereo effects you can manipulate things by flipping the sides (so that what was on the left is now on the right), or bringing the sides in towards the middle to narrow the stereo spread, or bringing one side to the middle to bias the sound off to the other side. Although it's useful to have overall ambience type sounds in stereo, you have a lot more freedom if you have the individual effects as mono elements and build up the mix with those elements panned to wherever works best in the overall idea.
I hope that helps. I can't really go beyond those generalisations without more information.