This question has been raised several times in the past.
From what I remember, the general advice was that the head unit is the guilty party , not the speakers, as the frequency response of the head unit is rather limited.
I had my Mk2 Jazz for 3 years and also intended to replace the speakers, easier than the head unit and bought some Alpine speakers, but never got round to replacing them as the sound quality gradually improved.
Car audio sites tell you to 'run in ' speakers for many hours to get the best sound quality-- I'm not sure I believed this , but I have to admit the sound quality was more acceptable on the standard unit after I had the car for a year, hence couldn't bother to replace the speakers.
The sound quality was certainly not brilliant, but acceptable for a car.
I would suggest waiting a bit, see if it gets better, and if not do a search on this and the Fit forum-- loads of stuff there.
I don't have the Jazz any more, replaced it with a Civic, and the sound quality is still not brilliant, but it's a car !