Can't get multichannel audio on emu10kx

# sysctl dev.pcm | grep EMU
dev.pcm.4.%desc: EMU10Kx DSP side PCM interface
dev.pcm.3.%desc: EMU10Kx DSP subwoofer PCM interface
dev.pcm.2.%desc: EMU10Kx DSP center PCM interface
dev.pcm.1.%desc: EMU10Kx DSP rear PCM interface
dev.pcm.0.%desc: EMU10Kx DSP front PCM interface
But sound is always down-mixed to stereo. rest of channels remain silent.

What should I do to get multichannel playback?

What application do you use for playback? It's possible it's the player that does the actual down-mixing.
According to man page, snd_emu10kx(4) driver has two modes, exposing channel pairs as separate devices or as one multichannel device. It was done in times when FreeBSD had no multichannel audio support. You may try to switch the driver into multichannel mode.

Also don't forget to switch virtual channels subsystem to wanted multichannel mode using sysctl(8) like"s24le:5.1"
I tried mpv, baka and many others - results the same.
According to man page, snd_emu10kx(4) multi-channel enabled by default. If I set hint.emu10kx.0.multichannel_disabled=1, I get only one device, and sound goes from all channels, even for stereo content, but all channels have the same content. Something still wrong.
hint.emu10kx.0.multichannel_disabled=1 in /boot/loader.conf helped after removing pulseaudio.

Thanks a lot for help!