Be sure to use a digital audio connection like HDMI, SPDIF or Toslink instead of the two rca cables for stereo.
Be sure to use an audio processing mode that is not distorting the sound. Typical modes that work well are Stereo (music only), Dolby Digital, DTS, Prologic IIx.
Be sure to select the highest quality audio track on a Blu-Ray or DVD. These are typically TrueHD, DTS-MA, DTS and Dolby.
Try placing the receiver at the default settings if you used the automatic setup program.