Adding more sound files to resource packs

Solution 1:

Yes, you are able to play custom sounds with Command Blocks using command:

playsound <sound> <player> [x] [y] [z] [Volume] [Pitch] [minimumVolume]

Minecraft Wiki has the following description:

Example: Sound located at .minecraft\assets\sound\mob\pig\death.ogg The value for sound would be written as mob.pig.death, as the command originates from the sound folder. For sequentially numbered files pertaining to the same event (like various walking sounds), the player must exclude the number of the sound (for example, sound located at .minecraft\assets\sound\mob\pig\say2.ogg). In this case, the command will play a randomly selected sound out of the three available. A player or area must be selected, and the player must be able to hear the sound from where it is played.

I have no knowledge on adding more numbered sounds, but the wording of the above description makes me think it should be handled OK by the game. Unless anyone else adds that information, you'll have to test yourself - just add a loud scream to zombie groans and walk around for some time :)