Half the battle with headphones tends to be isolation. If you have decent sounding speakers but bad isolation you have mediocre at best headphones. Typically replacing the ear cups can be an option but adding speakers likely won’t yield additional quality. Unfortunately if the speakers that already are in the headphones aren’t up to snuff there won’t be a whole lot you can do. It’s important to make sure we’re talking about the actual headphones though and not the audio amplifier or the source audio.