Things You Can't Hear.
Or rather, there's no air, so sound doesn't travel. Vibration has no medium to suspend or transmit it, so your screams go no further than your own helmet. Under nearly all circumstances, space will just be quiet.
Which means when a nearby ship explodes, you can't hear it.
When another ship passes near yours, you won't hear it's engines either, unless it's actually close enough for your ship to bump into them or their plasma jet, which would be bad. Considering how mind-bogglingly huge space is, there's no reason for ships to ever get that close to each other. But that's a topic for another page or two.
Even in the middle of a battle, with wreckage and weapon impacts in your field of view, some poor sod floating in a space suit will be struck by just how quiet and serene it all is. All you hear is your own breathing, and the sound of your own space suit.
Things You Can Hear
If an explosion happens on the ship you occupy, however, you will hear (and feel) that. In fact, sound (not just explosions, but voices, and certainly engine sounds) will carry and echo a lot inside a pressurized spaceship, because it doesn't have anywhere else to go.
A nearby Nuclear weapon explosion might be heard as well, but not for conventional reasons, since, again, there is no air between the ships. If the explosion is within a few hundred meters of your vessel, the flood of X-ray radiation the Nuke unleashes may be strong enough to "flex" the hull of your ship, which would likely cause a very unsettling sound.
In A Pinch
Spacecraft, especially battle-hardened spacecraft, are likely to have some back up methods. Directional transmissions and boosted signals are likely, and hardened broadcasts that are more difficult to jam.
Lights and morse code are a possibility, but battle-hardened craft will have few if any windows to see them from, and will likely fly at or beyond the limits of unaided sight. If the other ship knows your comm systems are down, it will probably be able to look for your flashing lights, but if your comm failure was unexpected, it might take them a while to notice. If all else fails, some course corrections should get their attention. There is no Stealth In Space, after all, and your stardrive is probably a giant nuclear torch. If you've got fuel to spare, you could even morse with your drive.
You might also send someone over in a space suit to bang code on the hull of their ship. Assuming it's not drowned out by their engines, they'll hear you going to town with a hammer.
For those in a EMU, or Orlan Space Suit, or something similar, whose radio or Snoopy Cap has gone out, there is a back up method for talking to a nearby astronaut. Lean the helmet of your spacesuit up against the helmet of their spacesuit, and then talk. The vibrations will pass from one helmet to the other if they are physically touching. Your voices will be muffled and distorted, but they should be understandable.
Game and Story Use
- Running a realistic space game may require adjusting the descriptions you give the players.
- When you do use sound as a description during a space combat, it'll be because of something big. Make it memorable.