Savage Worlds relies heavily on situational rulings by the GM (that's the entire basis of the Comprehensive Modifiers side bar on page 34).
Silence imposes a situational -4 / automatic failure on Notice rolls for sound; not all Notice rolls (it doesn't make observers blind), just Notice rolls for hearing.
Similarly, invisibility doesn't affect being heard or smelled or touched, just seen. So, the invisibility power affects Notice rolls that rely upon vision and attacks that rely upon vision to accurately locate the character.
These are different situations, so their benefits don't add together. Rather, the character is now extremely difficult to detect in two different situations - sound and sight. Assuming both powers were activated with a Raise, and that silence had the Mobile power modifier, then the character cannot be heard (all sound-based Notice rolls automatically fail) and is extremely difficult to see (-6 to visual Notice rolls).
Which is generally fantastic, but not any help against a bunch of monsters with a sense of smell so keen they can locate targets.
For the corner case where a character has invisibility and silence (with a Raise) active against alert observers, the situation will dictate the penalties for those observers. If they are only relying upon vision then their Notice rolls are -4/-6; if they are relying only upon sound then they automatically fail; if they are relying upon both then they are only relying upon vision - and modifiers from audio sensors, noise makers, or loud ground would be completely ignored.
A tactically more interesting use of silence is to silence the guards and then drop a grenade on them - they die and no one can hear it.
For completeness, I should note a commonly overlooked part of how the Stealth rules work.
Unless observers are actively alert, Stealth rolls are for a simple success. Even failure simply makes observers alert. Only alert observers get to try and Notice a Stealth roll (before the final step, as noted under Sneak Attack).
Sentries on alert for mundane troops should not count as alert when dealing with invisible or silent foes. If potential attackers are known to use invisibility or silence (or both) then the guards might be alert for such things.
I should also note that you can head to the PEG forums to use the Official Answers section and get an official answer from a PEG employee.
