If you add additional Custom sounds to your assets folder such as
sound/random/test.ogg and then use the /playsound it will play the sound
however, when you do this is a resource pack, you can only overwrite existing sounds, ( or mc is not picking up the additional sounds), one weird thing I did find out was:
you have multiple "random" sounds such as the Ghast scream1 scream2 scream3, but you can add you own scream4 to it if you want. and that will work, but still adding in custom additional sounds do not work