When I generate a text to speech file from http://www.fromtexttospeech.com/ and import the MP3 file into Kdenlive, the audio is automatically sped up. Why is this happening?

Just created a (german) file there. ffprobe <filename> reports some errors, e.g. "Error reading comment frame". And maybe more important: "Estimating duration from bitrate, this may be inaccurate."

And most important: Both, vlc and kdenlive play the mp3 file in 6,04 seconds, so my file is not pitched by kdenlive.

You may have more luck when converting the mp3 to wav or flac before using it in kdenlive.

I tried converting the mp3 file to wav and ogg formats with audacity but the sound clips were still sped up in Kdenlive. I ended up exporting the video without sound and using Avidemux to add the audio file to the silent exported video.

Weird - btw, which vesion of kdenlive do you use? Which OS/distro? If you do not use the latest version, you may want to upgrade and test again.