From a quick look at the source code (I very likely wrote this part, but it was also a very long time ago so I don't recall it too well), it appears that unlike graphics - which NeoLemmix is written to account for frequent changing of - sounds are very much treated on a basis of "load once, assume it won't change". For the same reason, if you were to replace a sound file while NeoLemmix is running, it would continue to use the old sound until restarted, provided that either the sound is one of the default sounds, or it's already been used at least once during the custom session. (On the other hand, if you replaced a graphic file, while it might not change immediately as NeoLemmix does cache graphics, it would eventually if the corresponding image isn't used by NL for a while.)
If a need ever arises to change the way the sound system works to be more like that of graphics, then at that point reimplementing per-pack success / failure jingles can definitely be considered, but doing that just to reimplement this feature would be a lot of work for a feature very few people use. The only reason I can think of why we might need to do that is to avoid excessive memory usage, but so far sounds have not really been a huge problem in that regard, graphics are (which is why graphics are discarded from memory when not used for a while).