SMPlayer 17.8 brings a new audio filter which makes the audio easier to listen to on headphones. You can find it in the menu Audio -> Filters.
This is the ffmpeg earwax filter. From the ffmpeg documentation:
This filter adds ‘cues’ to 44.1kHz stereo (i.e. audio CD format) audio so that when listened to on headphones the stereo image is moved from inside your head (standard for headphones) to outside and in front of the listener (standard for speakers).