Regarding the comments:
Hmm, did not encounter this issue before with the Wii. If you are sure that all the Wii settings are correct. It could be either the AV port (on the Wii it self) or the Sound IC. Both are difficult to replace. It's most likely that you do need to replace the Sound IC. Which can be found here: https://www.zedlabz.com/products/audio-video-encoder-u6-ic-chip-for-nintendo-wii-console-motherboard-ave-rvl-a-c8391-replacement-pulled-zedlabz?srsltid=AfmBOorpQnhhBvTWfDD1BCn22mf_w4nxK-kdRUcU8gPHfpL_HWU-9qdB&variant=31236347691110
Sorry for the bad news!