P25 Phase I uses the IMBE voice codec. DMR uses the AMBE+2 voice codec. Both voice codecs are manufactured by the same company, DVSI. P25 Phase 1 uses 12.5 kHz of bandwidth for audio. DMR uses 6.25 kHz bandwidth for audio. A lot of people like the sound of P25 better because it's using twice the bandwidth.
Have you heard the difference between System Fusion in DN mode (6.5kHz) and VW mode (12.5 kHz)? Same thing, VW mode is using twice the bandwidth. That's why VW mode sounds a lot better.