PCM 5.1 is good! Uncompressed 5.1 channel audio! It's actually the best of the three mandatory Blu-ray audio formats. The other two are DD 5.1 and DTS 5.1 (the same formats as used on DVD).
Although Blu-ray does support Dolby Digital Plus and DTS HD, these audio formats are not mandatory, and decoders for them may not be present on the first-generation players (Sony BDP-S1 in particular).