If you have a Windows notebook, it might have been shipped with some APOs (Audio Processing Objects) by the vendor. Typical names are “DolbyDAX” or “MaxxAudio”. These APOs “optimize” the sound by introducing filters and compressors into the signal chain to make audio sound “better” through your notebook’s speakers. But they also mess with headphone audio and lead to a severe degradation of the audio quality in my opinion. As far as I know, there is no user-friendly way to turn them off. And even if you manage to turn them off, chances are that they are back with the next bi-annual Windows 10 feature update.