I did something very similar with my basement theater. I have a large multi-purpose room down there that is approximately 20'x25'. I created the theater in 1/2 of the space, but wanted to be able to open up the space for parties.
I had the advantage that the main beam had a soffet already built at the 12' point. I was able to mount a rod along the side of the soffet. For the curtains, I used the Eclipse brand. These are sold by many of the home improvement stores. I purchased mine at Bed, Bath and Beyond as they were being closed out for $15/panel. I needed a total of 8 panels to properly cover the 20' span. I then sewed the edges of the panels together, so there was one split in the front and one in the back of the room. I sewed a contrasting ribbon to the gap point, so guests could easily find the openoing.
The Eclipse drapes are lined and thick. They advertise both light blocking and sound blocking qualities. It is not like having a dedicated wall, but is does block enough sound that my kids can play xbox on the other side of the curtain while I watch a movie. The light blocking is quite effective. The Eclipse curtain came in a length of 84". The side mount to the soffet allowed me to mount the curtain flush with the floor. If you did this, you would likely need to create some sort of vertical header to get the rod down to 7'.
Don't know if that is any help, but it is something that worked for me.