WARNING:SOUND: Only wav-files with 8 or 16 bit PCM format (format=1) supported, got format=65534 and bitdepth=16.
ERROR:SOUND: Failed to decode sound (-2)
ERROR:GAMESYS: A sound could not be played, error: -8. Message 'play_sound' sent from main:/instance5#script to main:/instance5#sound4.
What means those magic numbers in warnings and errors and what do I need to do to get rid of them?
The format=1 is relating to format this specification. 1 means PCM format.
If the format= 65534then it’s something wrong with the resource.
Double check the type of your source file first.
Thanks 
what I can add is, in those cases you can use https://online-audio-converter.com, it helped me 
Hate to awaken a dead thread but I have something to add.
For those who want to convert files en masse to a Defold suitable format, or who do not like/trust online converters when avoidable, a great solution is to use ffmpeg. Here is the specific command I use to convert files to a Defold suitable format:
ffmpeg -i {infile, any format} -ar 44100 -ac 1 -acodec pcm_s16le {outfile, .wav}
Since it is bash, you can easily automate it or integrate it with your other audio workflows.
The easiest way to install ffmpeg is homebrew. Instructions here.
I hope this helps someone. It would have saved me half an hour if I’d come across it in this thread rather putting it together myself.
Or use audacity and convert to ogg format with 44000 hz
But then you would have to manually do this for each sound which is something I wanted to avoid. Integrating with bash allows you to chain your workflows and make the development of new content for your game project much faster and less error-prone.