In our 5er, the combo DVD/CD/surround sound system was in a slot below the TV. It died. And the TV was an older digital but basically square screen. I installed a widescreen HD TV that was slightly larger than the opening, but being so flat I could mount it on the outside of the opening and still looked very good.
The slot below it limited the size of the DVD/sound system I could install. So I took measurements, went to Best Buy, looked at all their all in one home theater systems, measured each and found one that fit perfectly in the space available. They had several models from several different manufacturers. They all came with new speakers, so that was part of my decision also. New speakers were close to the original in size, but I did have to enlarge some cabinet openings a little. But better sound was worth it.
I spent a just little more and got a blu-ray DVD/surround sound system since we had a number of blu-ray movies already at home. Really was not much higher price.