i get no sound when playing or rendering .mp4s in kdenlive

not enough information...
What source material do you use?
Any samples?
What project profile do you use?

What is the audio codec used? MP4 allows different audio codecs.
Double-click the file in the Project Tree to open Clip Properties. Then, click the Audio tab to see what it says.

Also, play the mp4 with 'ffplay' command in a terminal window. If you get audio, try next with the 'melt' command also in the terminal window.
Thanks