If there is a title clip on the timeline with a simple-scroll.kdenlivetitle
is placed over a picture together with music, then the music will sound very distorted the more text scrolls by.
Also the speed of the preview becomes slower.

2 lines with 40 characters (font 60px)
is not yet effective, but
3 lines with 90 characters sounds distorted,
2 lines with 130 characters (font 41px) have an even more distorted sound.

i5-3570K all 4 cores are used, but no 100% load

Is the CPU too weak for this ?

AppImage 19.12.1, openSUSE 15.1, X11

the cpu/gpu/ram is completely not related to rendering output quality.
only thing that ram/cpu/gpu can do i to speed up rendering time.

i hope that you are talking about the final rendered videoclip. (is that true?)
because what happens on preview mode (Kdenlive is using just 1 thread for this) is related to file formats, complexity and a lot of other things, this mean that PREVIEW (is a quality less, frame skipping, timing imperfect PREVIEW of the final rendered output)

for example my xeon 12 threads with 32 gb of ram give me only 1 frame at second when overlapping 4 video clips with 3 composite effect at the same time on full HD. and well.. this is very annoying but is what can do just 1 thread ( 10% of maximum processor computing ) ..and i can hear an audio sound every 1 second...

no, I mean the preview mode in kdenlive.
And since I use version 19, all 4 cores of my CPU are used by kdenlive. Although only 1 thread is configured.
Version 18 only used one core and it was almost impossible to set a movie to music. The sound was extremely distorted.
The complete movie.
Unfortunately kdenlive (melt) used also only one core for rendering, no matter what I set.

With "taskset --pid --all-tasks --cpu-list 0-3 <PID>" I was able to change this for rendering and also for kdenlive, but the audio distortion didn't improve.
With version 19 it is now at least only in the movie credits. (I made a new version of an old 720p movie *.avi/*.mov)

If this works well with my 1080p movies (*.mp4), I can only test after the next holiday. The last one I created with v18.

I just wanted to know if I alone have the audio problem.

distorted mean that there is a frequency change? volume change?

https://youtu.be/P_QUrkwgZyc
that's a 720p format. Shortly before the end of the example, the playback speed also decreases.
When I use music it sounds stuttering, choppy.
In m last 1080p movie which I started with kdenlive v18 and completed with v19 the sound problems are the complete movie.
That's horrible!