I am trying to render a short video.
4 tracks: 2 video and 2 audio
profile I tried: H264/fast, AAC, (CBR) and MPEG-4/MP3

In all cases I do not get an audio stream but the output is a mute video with both the video tracks.
The error system informed me that
[libmp3lame @ 0x7f728c4c23e0] Channel layout not specified

What do you suggest?

what is your kdenlive version? which os are you using?