It does not say that your file does not exist - this message says that ash cannot find any executable in bin named foghorn1.wav, which should be run. The same way when you write eg. ls - ash then looks for a proper executable in bin and if no results are yielded, returns an error message.
If you want to run a file from current directory, you'd need to write ./my-file, but it only works for executables, which - clearly - a music file is not.
I dont see what the converter has to do with not finding and playing a file
You cannot play music without a music player, look at examples:
Nowhere there you'll find ./my-file.mp3 nor pure my-file.mp3.
You'd need to install some music player and hope it gets along with your converter.