"Mr. Fisher said Sprint would not comment on rumor or speculation."
Is an article missing between "on" and "rumor"?
No. Rumor (rumour - BrE) can be used countably and non-countably.
Context is always important; labelling is rarely important.
No, not necessarily.
a. There is rumour that he won't come back.
b. There is a rumour that he won't come back.
Both equally correct?
No.
Only b is correct. This is a single, specific, countable rumour.
There is also a set phrase meaning the same thing in which rumour is used uncountably:
'Rumour has it that he won't come back'.
Rover