According on my needs I use
1)
Dexed (VST plugin)
2)
DX7 Bank Individual Patch Extractor (and sender) (HTML tool by Retrokits)
3)
FMFriend (HTML tool by Foldplop)
1)
Dexed is a great VST plugin that allows you to
-
create/edit patches in a neat, visual way
- rename patches and organize them in banks
-
upload new patches to the Volca FM
Downside (if you don't use VST plugins already): you'll need to set up a VST host in order to use Dexed.
2)
If one just needs to
upload .syx patches into the Volca FM, it seems easier to use the
"DX7 Bank Individual Patch Extractor (and sender)" HTML tool by Retrokits.
(thanks spirit68 for
pointing that out). No setup needed (it runs in a web page), it only does one thing and it seems good at it.
Downside: what happens if Retrokits disables this page?
3)
FMFriend runs in a web page as well and offers a visual interface to
edit (almost in real time) all the parameters of the patch that your Volca FM is playing. No setup needed, so it's a very immediate way to get familiar with FM synthesis.
Downsides: it doesn't handle .syx files, so you can't actually upload patch files to the Volca FM. The web interface is a bit crude (but it's an open source project, so if you know some CSS/HTML and/or some UI things you may be able to help improving it)