Hi
You found a bug, nice one 
Ok, you’ll need to grab the updated function script, copy and paste all into term:
wget https://github.com/Fourdee/DietPi/raw/1d14025c3aece5d39465867baec4c63a4fc62767/dietpi/func/dietpi-set_hardware -O /DietPi/dietpi/func/dietpi-set_hardware
dietpi-services stop
/DietPi/dietpi/func/dietpi-set_hardware soundcard h3-analogue
dietpi-services start
Sound should now be functional through 3.5mm analogue.
Interesting, onboard sound supports 192khz/32bit output, but no where near the sound quality of Odroid HiFi shield / JustBoom card:
root@DietPi:~# cat /proc/asound/card*/pcm0p/sub0/hw_params
access: MMAP_INTERLEAVED
format: S32_LE
subformat: STD
channels: 2
rate: 192000 (192000/1)