Convert all stereo weapon sounds to mono, it fixes #2500

• Restore old shotgun melee sound: it's now mono but at least it has some decent low tones that were lost in the stereo version
• Restore previous mono weapon sounds that sound like the current stereo versions (fireball_prefire2.ogg converted from wav to ogg)
• Convert all (remaining) stereo weapon sounds to mono (keeping the original sound frequency since there's no quality gain in upscaling and the sound files are smaller)