You can find a more in-depth tutorial on custom sounds on wiki.bedrock.dev here.
Minecraft can read sound files in the .ogg(recommended) or .wav format. (You can look up an .mp3 to .ogg converter online). Personally, I usually get my sound effects from ZapSplat.com.
Sound files are located in subfolders of the res/sounds/ folder. My skele_yaklin sound files are located under res/sounds/mob/yaklin_moo.ogg and res/sounds/mob/yaklin_moo_2.ogg. (I don't have more sound effects currently, thus I will be using same ones over and over, but ideally one would need more of those).
Now to define the sound's shortnames! This process is similar to defining block texture shortnames. It's done in res/sounds/sound_definitions.json.