I'm documenting this question and answer for others for my specific situation that I hope is a benefit for others. I'm purposely being verbose in hopes that some sort of keyword search or general description will help more users find the solution.
I have a Dell Latitude laptop running Windows 10. After a recent software upgrade (it is likely the Spring Creators Update, but I'm not certain), audio coming from my speakers began sounding terrible. The bass was lacking, the audio was clipping, and it just sounded muddy, underwater, or like I was in a small enclosed room.
I tried different audio sources from different programs, but there was no difference. To isolate the issue and help with reproducing the issue, I chose to use a specific song played back using Foobar2000.
I tried different outputs from the same device - built-in laptop speakers, bookshelf speakers attached to a traditional external 2-channel amplifier powered by the headphone jack, a quality pair of headphones powered by the headphone jack, as well as the same external speakers via the USB-C docking station's headphone jack. Outside the difference attributed to the quality of the speakers themselves, the problem remained.
I also tried different devices. Luckily, I had another Latitude laptop running the same software to compare to, as well as an Android device with Google Play Music. The audio issue did not persist across devices, regardless of the speakers used.
That helped me rule out the music file, the playback software, and the output devices. It left me with the hardware or other OS software. However, since the audio was fine until a recent update and is generally left docked, I used some common sense to rule out the hardware as well.
After attempting to use the built-in audio troubleshooter (useless), trying different driver combinations, manually installing different driver packages, and even deleting the hardware devices (from Device Manager), there was still no change.
I checked with an audio engineer in the family, and they agreed the sound was like an equalizer had been applied to the system. But I had already ensured no OS-powered equalizer was running, nor was one configured in the DSP Manager for Foobar. My devices were all set to Stereo, not 5.1 Surround, so that wasn't the issue either.
So what was causing me to pull out my hair?