Canonically, this battle is supposed to be taking place in the upper atmosphere of Coruscant, not "space". Although the definitions of "upper atmosphere" and "space" are a little fuzzy around the boundaries. Once you get above about 100 km from the surface of a planet like Earth you're basically in space (in fact that's the usual definition). And even at significantly lower altitudes the air is so thin that sound won't really propagate easily. When Joseph Kittinger jumped from a balloon at 31,000 metres, he reported that there was no sound. (This is the highest ever parachute jump, and a fascinating story, I recommend you read about it.)

Oh, and yes, we know about the rationalisation of simulated sounds in the fighter cockpits from the surrouding battle: we mentioned it before in the annotation to strip #171.

Transcript

Anakin: This is where the funeral begins.
[SFX]: Pow! Pow! Pow! Pow! Kaboom!
[SFX]: Pow! Pow! Pow! Pow! Kaboom!
[SFX]: Pow! Pow! Pow!
Obi-Wan: Aren't we in space? How exactly are we hearing this?
GM: Uh, good point.
Anakin: Save your shouting for when Jim gets back.
R2-D2: This is eating me up. I have to ask. What happened with you and Jim?
{the space battle proceeds in dead silence}
{further silent battle}
GM: Yes. The silence of space sure is majestic.