I've managed to put this back into the old-formats versions even.
To put a custom jingle, include it as a custom data file and call them "success.ogg" or "failure.ogg" (or any other suitable extension; please note that only waveform based formats (OGG, WAV, etc) can be used, not module-based ones (MOD, IT, etc), and OGG specifically is recommended).
This will not work on the current versions of NL, but will not cause any problems on them either. For all future versions, it will work.