YES I noticed that my tablet is a "borderline case".
only by doing as I did I can make the audio work.
I think mine is a poorly designed tablet, in fact I have a Micro-USB port (otg ?),
which has never worked. I don't know what it is for.
if I connect it to the cumputer nothing happens.
it does not even recharge the battery because it has an external 12 volt connector.
[ 4.014046] bytcr_rt5640 bytcr_rt5640: BIOS Routing: AIF1 connected
[ 4.014064] bytcr_rt5640 bytcr_rt5640: quirk IN1_MAP enabled
[ 4.014067] bytcr_rt5640 bytcr_rt5640: quirk realtek,jack-detect-source 3
[ 4.014070] bytcr_rt5640 bytcr_rt5640: quirk realtek,over-current-threshold-microamp 2000
[ 4.014073] bytcr_rt5640 bytcr_rt5640: quirk realtek,over-current-scale-factor 1
[ 4.014076] bytcr_rt5640 bytcr_rt5640: quirk DIFF_MIC enabled
[ 4.014079] bytcr_rt5640 bytcr_rt5640: Invalid routing, bytcr detected but no SSP0-based quirk, audio cannot work with SSP2 on bytcr
[ 4.014081] bytcr_rt5640 bytcr_rt5640: quirk MCLK_EN enabled
[ 4.019134] bytcr_rt5640 bytcr_rt5640: snd-soc-dummy-dai <-> media-cpu-dai mapping ok
[ 4.019220] bytcr_rt5640 bytcr_rt5640: snd-soc-dummy-dai <-> deepbuffer-cpu-dai mapping ok
[ 4.021524] bytcr_rt5640 bytcr_rt5640: rt5640-aif1 <-> ssp2-port mapping ok
[ 4.025882] input: bytcr-rt5640 Headset as /devices/platform/80860F28:00/bytcr_rt5640/sound/card0/input1