1. The highest bitrate for Dolby Digital (in any channel configuration) on DVD is 448kbps. The recommended bitrate for 2.0 surround is 192kbps.
2. How's your maths? 48kHz (1000 samples per second) x 16 bits per sample x 2 channels = 1536kbps. I suppose you could use 96kHz/24 bit if you really wanted to...