There's an issue I'm having with mpd, Flash and alsa on Sid. I've yet to discover what the issue actually is, but it means frequent crashes when both mpd and Flash are trying to make noise. Usually it means Iceweasel or Epiphany crashes when some (but not all) flash videos try to make noise, occasionally it's the other way round and mpd crashes. And when mpd crashes, I have to stop Iceweasel, restart mpd and then start Iceweasel again. I've tried running strace on both programmes, but I learn very little I didn't already know - they hang in exactly the same way after trying to access /dev/snd/controlC0.

open("/dev/snd/controlC0", O_RDWR)      = 6
ioctl(6, USBDEVFS_CONTROL, 0xbfee5d88)  = 0
ioctl(6, UI_DEV_CREATE, 0xbfee5d30)     = 0
close(6)                                = 0
open("/dev/snd/controlC0", O_RDONLY)    = 6
close(6)                                = 0
stat64("/usr/share/alsa/alsa.conf", {st_mode=S_IFREG|0644, st_size=8611, ...}) = 0
open("/dev/snd/controlC0", O_RDONLY)    = 6
close(6)                                = 0
open("/dev/snd/controlC0", O_RDWR)      = 6
ioctl(6, USBDEVFS_CONTROL, 0xbfee5d88)  = 0
ioctl(6, UI_DEV_CREATE, 0xbfee5d30)     = 0
close(6)                                = 0
open("/dev/snd/controlC0", O_RDONLY)    = 6
close(6)                                = 0
stat64("/usr/share/alsa/alsa.conf", {st_mode=S_IFREG|0644, st_size=8611, ...}) = 0
open("/dev/snd/controlC0", O_RDONLY)    = 6
close(6)                                = 0
open("/dev/snd/controlC0", O_RDWR)      = 6
ioctl(6, USBDEVFS_CONTROL, 0xbfee5d88)  = 0
ioctl(6, UI_DEV_CREATE, 0xbfee5d30)     = 0
close(6)                                = 0
open("/etc/group", O_RDONLY)            = 6
fcntl64(6, F_GETFD)                     = 0
fcntl64(6, F_SETFD, FD_CLOEXEC)         = 0
_llseek(6, 0, [0], SEEK_CUR)            = 0
fstat64(6, {st_mode=S_IFREG|0644, st_size=964, ...}) = 0
mmap2(NULL, 964, PROT_READ, MAP_SHARED, 6, 0) = 0xb7f57000
_llseek(6, 964, [964], SEEK_SET)        = 0
munmap(0xb7f57000, 964)                 = 0
close(6)                                = 0
open("/dev/snd/controlC0", O_RDONLY)    = 6
close(6)                                = 0
semget(0x56a4d5, 1, IPC_CREAT|0660)     = 1212423
semctl(1212423, 0, IPC_64|IPC_STAT, 0xbfee6218) = 0
semctl(1212423, 0, IPC_64|IPC_SET, 0xbfee6218) = 0
semop(1212423, 0xbfee63e2, 2

And yes, it really does hang in the middle of that last semop. I'm now stuck as to how to debug this further, or even where to file a bug report.