Steps to reproduce:

1. Use fish as your system shell.

2. Use dired-do-async-shell-command on any file, and type e.g. mpv.

Fish interprets "&wait" as part of the file name, so mpv returns: "[file] Cannot open file '027.wav&wait': No such file or directory."

Solution:

In the function dired-shell-stuff-it, replace "&wait" with "& ". Then fish interprets the command correctly.