Hi, my uncle has SurroundSound speakers installed in his living room that are wired with a 5-color male AV cable.
This sounds like it may have been part of a proprietary surround sound system instead of a world-wide industry standard system... Or, it could be banana plugs. Or, it could be something completely different.
It is important to be very specific if you don't know what you are looking at. Take a photo, then post from your phone, or post a link. Find a photo online of the connections, then list the specific make/model of the speakers which are in place in the room. Specifics, specifics, specifics! This way we can help you get really accurate information that is actually useful.
They were plugged directly into his cable receiver box but he recently upgraded to a new one which does not have AV inputs.
This is very unlikely. VERY. It's okay though.
Cable companies don't provide surround sound systems. He very possibly had a home theater in a box kit, and it may have been super proprietary, as I said before.
Specifics matter.
He was told he needs a Digital to Analog Converter (DAC) but does not know which one to get, or which input to connect it to. Tried to attach pics but of course, file too large. Help? Thanks much!
You will want to find a way to get pics online and linked. What you are talking about sounds like it could be a few different things that are going on and any answers are just going to be complete guesses from whoever provides them. Myself included.
That said, a DAC is a digital to analog converter. It makes no sense to need this or do this at all for most setups.
If you have a proper 5.1 surround sound setup, then you should purchase a proper 5.1 surround sound A/V receiver with industry standard connections, and industry standard HDMI inputs on it. This way, you plug the speakers into the receiver, and then plug your sources into the receiver, and you get surround sound. There are tons of videos showing how to do this and plenty of reading about it on the Audioholics website.