You might want ...

• to tell what the program displayed. Your webservers log file should hold further information.
• use system instead of exec, since exec terminates the current process (might work, but the HTTP response might be incomplete; simple webservers might terminate).
• use an absolute path for mplayer and options to terminate the player when done.
• check permissions: does your webserver has access to the audio device and to the mp3-file?
• /test.mp3 is located at your computers filesystems root-dir, not your webservers root-dir (where /cgi-bin/... starts). You might use an absolute (filesystem) path too.
• consider security issues (e.g. restrict access to 127.0.0.1., check user input).
• ...