Here's how I've done the charging-up effect of the beam cannons for my FS2 mod:
When coding the start, travel or end effect in efx.ini or mod_efx.ini, each EFX block is comprised of a bunch of different types of effects like FLARE, STRIP, SNAKE, etc.
Most of these effects include up to four timing settings: Delay, Attack, Sustain and Decay.
- Delay: how much time goes by after invoking the weapon effect, before that visual effect part actually starts to show up
- Attack: the duration of the "build-up" phase, where the effect grows to its specified size
- Sustain: how long the effect stays at its specified size
- Decay: the duration of the "tear-down" phase, where the effect shrinks to 0 again (to avoid a sharp cut-off of the effect)
Now the Start and Travel effects of the weapon usually start exactly at the moment the weapon fires. The End effect will be started when the weapon hits, which would be instantanous for laser-type weapons or when the projectile collides with the target for projectile-based weapons.
To achieve a charging-up effect, you simply need to delay the Travel and End effects. Use the Start effect to create your charging-up and adjust the Delay of your other effects accordingly. For example, one of my beam cannon effects looks like this, omitting all non-related parameters:
// Start
EFX 1
FLARE
Delay 0
Attack 1.5
Sustain 2.5
Decay 3
FLARE
EFX
// Travel
EFX 2
STRIP
Delay 1.5
Attack 0
Sustain 2.5
Decay 3
STRIP
EFX
// End
EFX 3
FLARE
Delay 1.5
Attack 0
Sustain 2.5
Decay 3
FLARE
EFX
Notice how the delay of Travel and End EFX matches the Attack duration of the Start effect. Thus it appears as if the laser starts firing immediately after being fully charged.
Also keep in mind that you generally want a short delay, because the game will not account for these delays when calculating the caused damage. This means, that a laser immediately applies damage to a target, even though it appears to still be charging. This is generally not a huge problem, but you'll want to keep the delay as short as feasible.
For the appropriate sound I'd advise making a single sound file combining both the charging-up and the fire sound and simply include that in a SOUND block in the Start effect.