Audio and Visual Annotations
In the last few years there have been a number of tools developed to support Audio and Visual annotations. A/V was added in IIIF version 3 so it is a relatively recent addition and the tool support is rapidly improving.
Timeliner
Timeliner is an annotation tool for music education. It allows students and educators to describe the structure of a piece of music (or any audio), creating a hierarchy of parts and visualising the parts as nested bubbles.
https://cultural-heritage.digirati.com/our-work/timeliner/
Europeana Video Editor
Another powerful annotation tool for AV is the Europeana Video Player/Editor. As well as playing IIIF AV resources the EU Player also has annotation functionality that allows you to create general annotations and also video subtitles.
Annotations
Subtitle editing
AudiAnnotate
AudiAnnotate is a publication platform for audio transcriptions created by various existing audio tools like Sonic Visualiser. The idea is that you can use your existing tool workflow to create a transcription then upload it to AudiAnnotate. AudiAnnotate will then convert the transcription into IIIF annotations and use a IIIF viewer to displayer the results. You can see a list of projects here:
http://audiannotate.brumfieldlabs.com/
and one project example here:
AudiAnnotate uses GitHub for storage so it can be run as a free service.