Hello!
Yes that can be normal is something else like pulseaudio is using the device. Can you try using the plughw as follows …
amixer -c tegrasndt186ref cset name='z LO DAC Playback Switch' 1
amixer -c tegrasndt186ref cset name='z LOL Output Mixer L_DAC Switch' 1
amixer -c tegrasndt186ref cset name='z LOR Output Mixer R_DAC Switch' 1
speaker-test -D plughw:tegrasndt186ref,0 -c 2 -r 48000 -F S16_LE -t sine -f 500
Jon