Applescript Service fails because of missing constraints
Since it was requested in the comments (sorry it took that long), here's a working version of the script. The trick is to put the filename (which contains spaces) into double quotes, because those can be escaped.

on run

    if (get output volume of (get volume settings)) &lt; 50 then
        set volume output volume 50
    end if

    do shell script "afplay \"/Library/Application Support/HelloWorld/hello.mp3\""

    return
end run