Had a look at your youtube sound design replacement.
- pick smaller games like iphone games to sound design and compose to - this is the market you will most likely enter first off in the industry not an in-house job working on a game like BioShock. You started off with quite a difficult video to sound design to. If you don't have the right material to begin with - specially guns explosions and other sounds - then it's going to reflect badly on your presentation.
- use the existing smaller games as a guide to sound design and see if you can replicate their work and mix the sounds and music accordingly to work together.
The first game I ever did sound design for was a student project - I recorded and synthesized all the material myself. There were only 8 sounds but those were the hardest sounds to create from scratch I have ever done.
Learn the ins and outs of audio editing / processing and learn digital audio theory as well as audio theory.
Read books on mixing.
Bisect game videos and listen to the unique elements in the sound design, pay attention to the mixing, do background sounds duck when a weapon is fired, what sounds stand out, what are their volumes, what frequencies do they use.
Sound design is a lot more than creating a few SFX and throwing them in the game - thinking about aesthetic choices and tonality, eq,stereo placement .. and psychology of the sounds are very important.