I'm not up-to-date in this area: Which applications are currently best for doing online presentations (slide shows) where participants may use voice to ask questions or discuss as a group and each event, both audio and slides, can be recorded for use later?

This, most inconveniently, must also be available for legacy operating systems, also.

Mumble is audio only. Could it be synced with slides somehow?
Tox looks promising but may not be that complete and the web site does not seem to have a summary of what it actually does rather instead focuses on why it is written. Jitsi seems quite rough around the edges, does the video enable use of slides? What am I missing with these or others?

we use skype on ubuntu and windows. But I'm not really sure it is the best. Usually works.

I can't tell if you are joking or not. Normally your posts are serious. I've used Skype before and it is not a serious option, especially not on GNU/Linux. The quality is just not there, even back before it was purchased.

What open source options are there? Failing that what options are there that at least subscribe to open standards?

We use Zoom at my company - it works perfectly in Linux. Most of my colleagues use Windows and it works fine on 7 thru 10 and it also works on Mac, should you wish to use it. Screen or application sharing is great, voice is great and people can also phone in if they do not have a mic. I also use the Android app. It also functions as a useful instant messaging client.

Not sure which parts of the service are free (the software is free). As we use it in a corporate capacity, meeting rooms have some fancy stuff set up with giant screens and tablets to control things, but I like to work from home mostly!

no, it is not a joke. Skype is _the_ official tool. I think it is not the skype we know, because it is now owned by MS and integrated into MS Office (and therefore it can work together with outlook).
And obviously it is not really perfect. Just it can do online video conference with presentations.