Any audio coming from directly behind you can not be heard which is absolutley stupid. If someone is shooting directly behind me I should be able to hear it! This needs to be addressed.
There are some suggestions around these issues in this article on the Halo Support site:
https://support.halowaypoint.com/hc/en-us/articles/4410860512788-Audio-troubleshooting-for-Halo-Infinite