An interesting take at AudioVisual art creation, free & opensource.
Will be extra interesting once they integrate Clock/Gate/CV input, which apparently they are working on. Looking forward to combine this one with VCV Rack, especially the video part! MSDP website
free, opensource, GPL 3
Link is dead? Weird. Can’t connect to the site, but downforeveryoneorjustme says it’s there. 
There is another one here https://www.musicsdp.com/ that works for me.
MSDP is built using the latest version of Max. If you want to edit the code you will need to have an authorized copy of Max installed on the computer you’ll be working on. The code can be viewed without authorizing Max, but the language will still need to be downloaded from www.cycling74.com. Editing will be disabled until a license has been purchased and your copy of Max has been authenticated. Note that even without buying Max, the patches are still usable identically to the application.
Thanks Latif, definitely worth a look … (downloads seem to be slow atm) Oh well …
Welcome Erik!
This looks pretty interesting. It may be a little too “regimented” for my tastes but any new FLOSS audio software is great.
It’s technically FLOSS, but requires buying and activating Max to build it. So it’s difficult to make good on the FLOSS aspects. 
Probably aimed at Art school students and so.