default or selected kernelversion does not have config value CONFIG_SND_SOC_RT9123.
Result is shown for kernelversion 6.16-rc7

Richtek RT9123 Mono Class-D Amplifier

modulename: snd-soc-rt9123.ko

configname: CONFIG_SND_SOC_RT9123

Linux Kernel Configuration
└─>Device Drivers
└─>Sound card support
└─>Advanced Linux Sound Architecture
└─>ALSA for SoC audio support
└─>CODEC drivers
└─>Richtek RT9123 Mono Class-D Amplifier
In linux kernel since version 6.1.145 (release Date: 2025-07-14)  
Enable support for the I2C control mode of Richtek RT9123 3.2W mono
Class-D audio amplifier.

source code:
sound/soc//codecs/