All Sonic Visualiser developer resources are now hosted at Github.
Please use the Github issue tracker to report bugs and suggest features.
There are older bug trackers hosted at SourceForge (bugs, features) and the SoundSoftware site (bugs and features) which are still lightly monitored. If you are thinking of reporting something, check first whether it has already been filed in one of these places — but it's best not to file new reports in these.
Do be aware that Sonic Visualiser is a mature project with relatively little developer resource, so stability and continuity are generally the priority. If you would like something done to a deadline or for a specific purpose, Particular Programs Ltd may be able to help you on commercial terms.
There is a Sonic Visualiser project on the SoundSoftware site. This is not currently being updated, but there is still some material there of interest.
A developer mailing list for Sonic Visualiser exists at SourceForge, but it is not in very active use. See information and subscription options and the list archive.
See the Vamp plugin developer page for resources about plugin development.