Best practices to teach while wearing a mask
I grew up in Canada and was a PhD student and lecturer in England, before moving to Japan from 2013-2016. Before 2013, like most Westerners I did not wear a mask during flu season, and couldn't imagine wearing one while teaching. When I arrived in Japan in 2013, there was no COVID-19 or SARS-2003 going on, but people everywhere were wearing masks: On day 1 people were wearing masks at the airport when I landed, at the convenience store where I asked for directions when I was lost, and in the audience when I gave my first talk at Kyoto University.

&quot;My main concern is my voice may be muffled&quot;

Don't worry: People have been teaching with masks for decades in East Asia (I witnessed it myself for 3 years in Japan and know from my colleagues that it was nothing new).
My advice for best practices:

First try to get used to wearing a mask. If you're like me, I had never worn one before 2013 and wasn't used to it right away. But it's like wearing glasses: it's a bit annoying to see the side arms of the glasses at first but after wearing them everyday for years you don't notice that anymore. Maybe wear a mask when you go out to the store, between now and Fall 2020, so that when you start teaching you are not uncomfortable.
Get a mask that's comfortable for you to be speaking loud for long periods of time. When teaching we often have to project our voice so that people can hear us in the back, which means we speak louder than normal, and might be moving our jaw more than normal. A lose mask that falls off or moves around if you move your jaw bones too much, might be a bad idea. You can try various masks before Fall 2020 to find which one you'll be comfortable in. This is the type I use and it's the type they were wearing all the time in Japan:


(source: homedics.com)
I got 50 of them for $30 in Canada with inflated COVID prices (they were cheaper before). I don't find myself needing to speak louder, and my voice is pretty much the same as without a mask (if you stick with this classic mask instead of trying to use an N95 or some t-shirt fabric turned into a mask, I think you'll be fine and don't need a visor).

The first comment pointed out that if you use a microphone between your mouth and mask your letter &quot;p&quot; can sound bad. I agree and I never wear a microphone when teaching, and in fact I ask the conference chair to turn down my microphone when I'm giving recorded talks at conferences, because early in my career people told me I was too loud. For centuries we didn't have microphones and lecturers like Richard Feynman didn't use one in lectures like the one in this link, so getting used to projecting your voice is part of teaching (even at conferences they don't always have microphones). If you do still prefer wearing microphones for some reason, I think you are teaching at a rather huge lecture hall where the students will be more than 1 meter away from you, so you are likely not to need a mask. Your question says &quot;Assuming that social distancing will not be possible while I'm teaching at the white board&quot; which makes me assume you're in a small enough room that you don't need a microphone.

If you absolutely must record your lecture, notice that the microphone in Ilya Kuprov's 900+ youtube lectures is nowhere near his mouth/mask. You can use a body-microphone or put it on a microphone stand somewhere.