Using the code below in cmd worked, of course you have to install ffmpeg, Thank you @rogerdpack for posting the link of the other stack, where I found solution link of the alexa docs https://developer.amazon.com/de-DE/docs/alexa/custom-skills/speech-synthesis-markup-language-ssml-reference.html#h3_converting_mp3
ffmpeg -i <input-file> -ac 2 -codec:a libmp3lame -b:a 48k -ar 24000 -write_xing 0 <output-file>