My husband cannot hold a conversation with anyone that they would understand. Even I do not know what he is saying. Is this common?

Imho, yes, this is a common occurrence with someone who suffers from Alzheimer's.

Yes. It happens when the parts of the brain that controls language are damaged. Sometimes people can still make their needs known non-verbally when verbal skills are beyond them.

Yes, this is a common symptom of dementia, usually occurring in the later stages. However, don't let that dissuade you from talking to him. A conversation no longer seems possible, but he can hear you, he knows you're there with him, he may recognize your voice. It's time to practice your monologues. I often spoke to my wife, while holding her hand reassuring her of my presence, about family issues, remembering vacations, even mentioning what will be for lunch. Remarkably, you may even hear him utter a clear discernible phrase. That, too, is normal and is not a sign of improvement. If it gets too frustrating for you to listen, you might distract his “train of thought” ( unfortunately, there is no train of thought, just some words he's trying to form without knowing how to put them together) by suggesting a snack or “how about a cup of coffee” or “I've got to do some laundry”, or anything else to interrupt his speaking.

I hope so. My dad says words I understand but without any discernible logic and context gets very frustrating for both of us. I think someone told me to stop trying to understand the logic of a person who has none.

My hubby has lots of trouble word finding. It is very hard at times to figure out what he means. It took a couple days to figure out that "backgrounds " are paper plates, "tablets"
are either cookies or slices of bread.
Mout of the time I just say Hmmm, or Uhhuh, just to show I'm listening.
The strange thing is when he talks in his sleep, and he carries on long conversations, he doesn't have any trouble with word finding.

Poster already gave the information: "I am caring for my husband Allen, who is 75 years old, living in my home with alzheimer's / dementia, incontinence, and parkinson's disease."

Yes, it is common for people with Alzheimer's to become completely unable to hold a conversation. It is sad to watch.

My mom will mumble at the end of the day when she has been up for 6 to 7 hours. She has been diagnoised has severe. I am sure my mother will eventually be unable to speak once this horrible disease continues to spread in the brain. I tell my mom when she mumbles "yes" or "you're right" or "okay" or "I will"

With more advanced Alzheimer's, yes it is very common for folks to start to mumble and not be understood. That is the brain affected by the Alzheimer's starting to affect the part of the brain that controls the speech.
I have a dear friend whose wife is in her 16th year of Alzheimer's and when he goes to visit her now, she just makes noises and mumbles. He just talks to her and no longer tries to figure out what she's saying, as it's pretty much impossible.
I'm sorry you are having to deal with this, but please don't let it stop you from talking to him, as he's still in there, and I'm sure loves to hear your voice and know that you're there for him no matter what. God bless you.

Need much more information.

If this is came on suddenly, get him to an emergency room immediately. He may have had a stroke.