What does "Mix Down" mean when choosing audio track for multicam sequence?

The Audio Track Channel options (including Mixdown) are not related to what audio is used in the multi-camera source sequence—this is controlled by the Audio Sequence Settings and the order in which you selected your files. I made this video to explain how this works: https://youtu.be/M89zzmK_qcc

The Audio Track Channel setting controls which audio channel of each file is analyzed to find sync.

• The options 1 and 2 mean Premiere will only analyze the first or second channel of each file. This is ideal if you have sync audio on the same channel for each file, and you want Premiere to ignore any other channels.
• Mixdown will sum all channels in each file and use this to find sync.

Mixdown is used when you have sync audio on different channels for different files (e.g., one file has the sync audio on channel 1 and the other file has it on channel 2) or when you have files that have sync audio on a channel other than 1 and 2.

The only drawbacks of using Mixdown are that the more channels your files have, the more processing Premiere will need to do before it finds sync, and if there is non-sync audio on the other channels that is loud, it may throw off its ability to find sync.

I have multiple camera and audio files and creating multi cam sequence to sync.

The footage will not sync unless I select "mix down"

When I open the generated multi cam sequence , all audio tracks are on the sequence however only track 1 plays, regardless of muting individual tracks or not.

does anyone know why this is ? 

I was able to copy and paste the multi cam sequence into a new sequence and all audio tracks worked and played back fine... 

So "mix down" definitely does something ...

Thoughts ? 

So you have two cameras, and a separately recorded sound-only file, right?

Select the three files, and whether you take camera 1 or 2 , Premiere will take that camera's sound, compare with the separate audio, and match both cameras to that separate audio.

It will put the specified camera audio and the separate audio on two separate tracks.

Mix down ... never used it! Even Jarle Leirpoll's amazingly detailed 1200+ page tome on accelerated professional use of Premiere doesn't cover it in his extensive Multicam chapter. He suggests for the basic use, just the general "automatic" option and probably Camera 1. Which will do as noted above.

For something like this, the easiest way to understand is simply do a test. Can't hurt or break anything. Select "mixdown". See what happens, then after figuring out what it did, just delete that multicam.