I have compiled mplayer quite a long time ago and probably used the daily CVS snapshot. Everything was fine up till today - when I tried to play one file.

$ mplayer a.ogm
MPlayer 1.0pre7-3.3.5 (C) 2000-2005 MPlayer Team
CPU: Intel  (Family: 6, Stepping: 6)
Detected cache-line size is 64 bytes
CPUflags:  MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled with runtime CPU detection - WARNING - this is not optimal!
To get best performance, recompile MPlayer with --disable-runtime-cpudetection.

Linux RTC init error in ioctl (rtc_irqp_set 1024): Permission denied
Try adding "echo 1024 > /proc/sys/dev/rtc/max-user-freq" to your system startup scripts.
Playing a.ogm.
Cache fill:  0.00% (0 bytes)    [Ogg] stream 0: video (FOURCC DX50), -vid 0
[Ogg] stream 1: audio (format 0x2000), -aid 0
[Ogg] stream 2: subtitles (SRT-like text subtitles), -sid 0
Ogg file format detected.
VIDEO:  [DX50]  720x400  24bpp  23.976 fps    0.0 kbps ( 0.0 kbyte/s)
==========================================================================
Trying to force audio codec driver family libmad...
Opening audio decoder: [liba52] AC3 decoding with liba52
Using SSE optimized IMDCT transform
[Ogg] Language for -sid 0 is '-slang "English[eng]"'
AC3: 5.1 (3f+2r+lfe)  48000 Hz  448.0 kbit/s
Using MMX optimized resampler
AUDIO: 48000 Hz, 2 ch, s16le, 448.0 kbit/29.17% (ratio: 56000->192000)
Selected audio codec: [a52] afm:liba52 (AC3-liba52)
==========================================================================
vo: X11 running at 1280x800 with depth 16 and 16 bpp (":0" => local display)
==========================================================================
Opening video decoder: [ffmpeg] FFmpeg's libavcodec codec family
Selected video codec: [ffodivx] vfm:ffmpeg (FFmpeg MPEG-4)
==========================================================================
Checking audio filter chain for 48000Hz/2ch/s16le -> 48000Hz/2ch/s16le...
AF_pre: 48000Hz/2ch/s16le
AO: [oss] 48000Hz 2ch s16le (2 bps)
Building audio filter chain for 48000Hz/2ch/s16le -> 48000Hz/2ch/s16le...
Starting playback...
mplayer: ad_liba52.c:315: decode_audio: Assertion `len <= maxlen' failed.

MPlayer interrupted by signal 6 in module: decode_audio
- MPlayer crashed. This shouldn't happen.
 It can be a bug in the MPlayer code _or_ in your drivers _or_ in your
 gcc version. If you think it's MPlayer's fault, please read
 DOCS/HTML/en/bugreports.html and follow the instructions there. We can't and
 won't help unless you provide this information when reporting a possible bug.

MPlayer interrupted by signal 11 in module: uninit_acodec

MPlayer interrupted by signal 15 in module: uninit_vcodec
Killed

I thought that the problem will disappear when I use the current version (straight from CVS repository) and I was right.
Additionally I have learned (thanks to google) solution for the older version of mplayer:

mplayer -channels 1 a.ogm