no sound via HDMI cable

hatahata

hi , all .
i use arch via emmc  on plus 2 , and it runs well .

perhaps this is FAQ .
i read  https://www.raspberrypi.org/docu ... ion/audio-config.md .
and
i do ' amixer cset numid=3 2 ' , but there is no sound via HDMI cable .

and thre is no  config.txt .

what should i do ?

ps) sound is gained by 3.5mm Jack , but this is unconvinient for me .
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regards tuyosi