Ran an alsa-info to try to get more insight into what the audio chip is doing. Not sure what to make of the unknown pin headphones messages. Perhaps someone could compare my result to their working setup:
!!ALSA/HDA dmesg
!!--------------
[ 1.829446] rockchip-saradc ff100000.saradc: Looking up vref-supply from device tree
[ 1.830173] usbcore: registered new interface driver snd-usb-audio
[ 1.834781] u32 classifier
--
[ 1.979831] rockchip-drm display-subsystem: bound ff8f0000.vop (ops 0xffffff8008d45a28)
[ 1.980583] dwhdmi-rockchip ff940000.hdmi: Detected HDMI TX controller v2.11a with HDCP (DWC HDMI 2.0 TX PHY)
[ 1.983345] rockchip-drm display-subsystem: bound ff940000.hdmi (ops 0xffffff8008d3ab40)
--
[ 2.667230] rockchip-dmc dmc: could not find power_model node
[ 2.673873] of_get_named_gpiod_flags: parsed 'simple-audio-card,hp-det-gpio' property of node '/rt5651-sound[0]' - status (0)
[ 2.673888] of_get_named_gpiod_flags: can't parse 'simple-audio-card,mic-det-gpio' property of node '/rt5651-sound[0]'
[ 2.692154] asoc-simple-card rt5651-sound: ASoC: DAPM unknown pin Headphones
[ 2.693891] asoc-simple-card rt5651-sound: rt5651-aif1 <-> ff880000.i2s mapping ok
[ 2.693959] asoc-simple-card rt5651-sound: ASoC: no source widget found for MICBIAS1
[ 2.693972] asoc-simple-card rt5651-sound: ASoC: Failed to add route MICBIAS1 -> direct -> Mic Jack
[ 2.694322] rt5651 1-001a: ASoC: mux INL1 Mux has no paths
--
[ 2.694420] rt5651 1-001a: ASoC: mux INR2 Mux has no paths
[ 2.698559] input: realtek,rt5651-codec Headphones as /devices/platform/rt5651-sound/sound/card0/input4
[ 2.699560] of_get_named_gpiod_flags: can't parse 'simple-audio-card,hp-det-gpio' property of node '/hdmi-sound[0]'
[ 2.699569] of_get_named_gpiod_flags: can't parse 'simple-audio-card,mic-det-gpio' property of node '/hdmi-sound[0]'
[ 2.700408] asoc-simple-card hdmi-sound: i2s-hifi <-> ff8a0000.i2s mapping ok
[ 2.706177] of_get_named_gpiod_flags: can't parse 'simple-audio-card,hp-det-gpio' property of node '/spdif-sound[0]'
[ 2.706203] of_get_named_gpiod_flags: can't parse 'simple-audio-card,mic-det-gpio' property of node '/spdif-sound[0]'
[ 2.707313] asoc-simple-card spdif-sound: dit-hifi <-> ff870000.spdif mapping ok
--
[ 2.735835] vcc_lcd: disabling
[ 2.737442] ALSA device list:
[ 2.737448] #0: realtek,rt5651-codec
--
[ 2.774218] mmc_host mmc2: Bus speed (slot 0) = 150000000Hz (slot req 150000000Hz, actual 150000000HZ div = 0)
[ 2.842491] asoc-simple-card rt5651-sound: ASoC: DAPM unknown pin Headphones
[ 3.011496] wifi_platform_bus_enumerate device present 1
--
[18640.323785] [UFW BLOCK] IN=eth0 OUT= SRC=192.168.2.107 DST=224.0.0.251 LEN=32 TOS=0x00 PREC=0x00 TTL=1 ID=56020 PROTO=2
[18688.338071] asoc-simple-card rt5651-sound: ASoC: DAPM unknown pin Headphones
[18695.449217] asoc-simple-card rt5651-sound: ASoC: DAPM unknown pin Headphones
[18696.131221] asoc-simple-card rt5651-sound: ASoC: DAPM unknown pin Headphones
[18699.250179] asoc-simple-card rt5651-sound: ASoC: DAPM unknown pin Headphones