Thank you for the bug report, Gullars!
I must have tinkered with the music playback speed years ago when I was still thinking of doing a remastered soundtrack version, and failed to change the tempo of the battle music restore code to the proper value.
Please install the new patch v3.36 in the download section to correct this bug. Unzip the patch file contents directly into the main Everlong folder to overwrite some old files. Once back in-game, open and close the main menu to reset the battle music to the correct speed.
Let me know if you encounter any more bugs as you play through the game! It's been a few years since the last patch, but I still have all my tools ready to fix any issues.