Some notes:

If you have ALSA version 0.5, then you almost always have to use -ao alsa5, since ALSA 0.5 has buggy OSS emulation code, and will crash MPlayer with a message like this:

DEMUXER: Too many (945 in 8390980 bytes) video packets in the buffer!

If the sound clicks when playing from CD-ROM, turn on IRQ unmasking as described in the CD-ROM section.