I use a sub-woofer in the sound system for all gaming. I found the shifting sounds for the Boxer to be very annoying. Especially during long races. Here I have made available the modified sound files for the Boxer car. Much subdued sound and can hear other sounds better now.
The folder downloaded is named: BoxerCup and it gets copied into C:\Program Files(x86)\Steam\SteamApps\common\Automobilista\GameData\Sounds\ overwrite when asked.
Be sure to backup the BoxerCup folder to another location in case the sounds are not satisfactory.