Minecraft version: 1.16.1
Forge version: 32.0.108
Environment: Singleplayer

Mod name: omegamute
Mod version: omegamute_1.16-1.3.jar

When I do /omegamute listen it doesn't register the sound of a minecart with hopper and I can't mute it.

Have you tried muting the following sound events?

entity.minecart.inside
entity.minecart.riding

Search for them in ./config/omegamute/soundmap.txt and replace them with

!entity.minecart.inside
!entity.minecart.riding

to mute the sounds.

Reload the config file by restarting Minecraft or running the command /omegamute reload to apply the changes.

how can I find ./config/omegamute/soundmap.txt
and when I do /omegamute mute entity.minecart.inside and entity.minecart.riding, I can still hear them

In this case, ./config means to go to the config folder of the Minecraft instance you are running.
The rest of the path should be self-explanatory. See the FAQ for more information.

nope it doesn't work