From testing V11.0.1054 on Mac OS 27.0.l with any sound file type its nothing to do with the sound type.
Its a bug with the location path to the file where eM Client loses the path after saving Settings as @Steve16 advised in his post at the top of this thread as per the below example screenshots where i used a eg: .wave sound.
Note: This is only an issue with the new mail incoming sound but the send mail file path location stays saved correctly and works ok.
