What onboard audio or sound card (if you have one) does your system have? Global system sound problems are typically adjusted in your sound device's control panel via a built-in Equalizer.
This for instance is what my Realtek HD Audio Manager looks like on my ASUS Prime Z370-A MB. If you are using onboard sound and don't have such a panel, make sure you check for the audio driver on the product page of your MB. Realtek is the most common onboard sound on MBs, and the drivers of the last two versions of it are only available on the product support page of your MB manufacturer.
That said, if you're savvy enough at using OBS to dabble into it's advanced settings, then you may want to use one of the many audio filter plugins for it. The common ones are compressors to mute unwanted sounds from your mic, but there's also at least one equalizer filter I've seen. These are definitely better for addressing things like mic audio and recording audio problems, but much more complex to use.
Note these Reaper VST plugins are made specifically for OBS
Download
http://reaper.fm/reaplugs/
Installation
https://www.youtube.com/watch?v=LD6_e13-sJ0
ReaEQ Guide
https://www.youtube.com/watch?v=oWiAp8sMTDg
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