How do I check to see if something specific is in workspace?

Use :FindFirstChild(). Like this

if workspace:FindFirstChild("Part") then
	print("A part exists in the workspace!")
end

To play a specific audio for only a client, do this in a localscript.

local AudioId = 1234567 --Insert audio id here
game:GetService("ContentProvider"):Preload(AudioId) --This preloads the audio, so the player can always hear it.
game:GetService("SoundService"):PlayLocalSound(AudioId)

Otherwise, to play a sound everyone can hear, put the script in the startergui. Also, I'd like to see you provide a script that you need help with next time.