Hello Gunner, nice to meet you. Ive replicated the bug on my end and recorded a video with sound on it. Since I can't upload it here due to the weight of the file Ive uploaded it to youtube.
https://www.youtube.com/watch?v=iwxu45OPROM
Now I will describe what's going on so the video so we can see if this is on my end or something.
I show you the audio which is all the way up to 100% and do a quick buy on the guild store. After that we proceed to the battle and get the ultimate going on.
The moment the ultimate is used, the sound of the music is noticeably diminished. As you can see, when a ship explodes, no sound is heard anymore, but some other aspects of the audio are fine.
On the victory screen and the menu after the audio is still low, I show you the audio volume again and make a purchase which makes the usual sound but the music is still a bit muffled. I eventually get into a squad battle and some things are still weird from an audio perspective.
Ive reinstalled the game a few times and play with a custom resolution of 1807 x 1016 in case it has something to do with it, V-Sync off and Anti-Aliasing on.
If there's no solution to it then there's no problem, I thought this affected more people.