Good eye spotting the mixed wire types. There is only one case where you'd ever expect to see that: individual wires in conduit. Given that this is a plastic box, the chance of this involving conduit is practically nil. This type of box doesn't even come with knockouts for conduit.
The blue plastic box suggests newer than 1989, so the 3-prong dryer receptacle should have been outlawed at time of installation. It has no ground, and worse, it relies on neutral for ground, so if the neutral breaks, it energizes the chassis of the dryer! This has killed people.
What most likely happened is someone ran /2 cable, hoping to get away with that (H1=black H2=white N=bare) but found out that is not legal, so they got a loose red THHN wire and ran it along with the cable. That is illegal too.
First a THHN wire isn't armored, but mostly, all wires in a related circuit must be in the same cable or conduit. That is because they kick considerable electro-magnetic fields. When they are bound together, those fields cancel each other out. When not bound, they vibrate and cause metal fatigue, or heat metal things between them (staples!) Either one can start a fire.
There's also a fair chance he used a #12 or #14 wire instead of the #10 appropriate for a 30A dryer circuit. If so, that would also result in wire burn-up.
Regardless, your circuit now appears to be dead, possibly because that metal fatigue cracked a wire, making a hotspot that burnt up.
The "50 volt" reading is phantom voltage picked up capacitively from the nearby hot. The wire is disconnected and floating. The 50V doesn't have enough strength to do useful work, not even move a mechanical meter movement in an old school voltmeter. (cheapie DVMs are sensitive enough).
What to do
It's time to replace the cable with a proper 10/3+ground cable, then replace the socket with a NEMA 14-30, and replace the plug if needed. You might find a ground wire inside the box, but the hokey-jokey arrangement with the oddball wire must not continue!
It happens all the time that someone acquires a used dryer (or buys from a lazy appliance store) that has a 3-prong plug, but their socket is 4-prong. No big deal, it's easy to swap a cord to 4-prong. But something possesses them to change the socket to an obsolete type. Maybe they fondly remember being shocked by their dryer in their youth. Maybe the socket was $2 cheaper than the cord?