You may well find that the reflective properties of the plasterboard make for a rather unpleasant and harsh-sounding room, which is not really what you want if you're going to be recording in there, and certainly not what you want if you're going to be monitoring and mixing in there either.

Simply having the room carpeted and putting a sofa at one end will probably help, but to address the wall reflection you can use a combination of two solutions: absorption and diffusion.

Diffusion involves sending the reflections off in different directions, breaking up the sound, while absorption soaks up certain unwanted frequencies. This is where the old eggbox myth can be laid to rest – while their shape should make them potentially good sound diffusers, their material isn't reflective enough, and its absorptive properties are minimal too. They don't look very nice either!

So, try picking up some 2'x4' sheets of hardboard and make yourself some absorption panels using 1" slabs of mineral wool (the same size) placed over the top and a dustsheet laid over and tightly stapled from behind. They'll look better and, more importantly, will actually work when hung on opposite walls.

If you'd prefer to invest in something ready made for the job, try some Aurelex, ProFoam or similar. These companies make a range of products for studios and even make room packs with all the different elements you'll need for different sized rooms. As you'd expect, this comes at a price, but gives professional results both sonically and visually.