In 1961, the Soviets exploded an H-bomb with an estimated yield of 50-60 megatons of TNT, or roughly 3,000 times the raw power of the Hiroshima bomb. Kruschev made noises about that time to the effect that they were developing a 100 MT bomb, but we’re talking about a guy who once took off his shoe and banged it on the podium at the UN to make a point. Current stockpiles consist mainly of much smaller warheads, mostly 1 MT or less, the theory being that you can have a lot more fun with ten little ones than you can with one big one.
An incoming ICBM is something you’ll probably never see or hear. As the name “ballistic” implies, these missiles are unpowered for most of their flight, so there’d be no engine noise. Even if there was, the missile would be traveling at high Mach numbers and would get to you long before the sound did. During WWII, Londoners hated and feared the V-2 for that very reason – you couldn’t hear it coming, and it moved so fast that seeing it wasn’t much help.
With the fissionable materials available, there is a fairly fixed upper limit to the potential yield of fission weapons (“A-bombs”), for practical mechanical reasons. However, the sources I’ve seen generally agree that fusion weapons (“H-bombs”) are unlimited in that respect. Most “H-bombs” are actually combinations of fission and fusion devices, using a fission bomb to generate the heat necessary to set off a fusion reaction, which then sets off a second fission reaction. Triple whammy, so to speak.