I realise that the sound cuts out as I mentioned but it does not whilst you press fire and shoot in your ship at the same time with the mouse.

It's not because you use the mouse to shoot, and it's not the file format that matters. It's the routine I'm currently using.

Right now, whenever you press a number key while aboard a vehicle, the following happens:
- a command is issued to stop every sound
- the default volume of the vehicle sound is modified, according to what key you pressed
- the vehicle sound is played at the new volume, looping

I explicitly issue the command to stop every sound, because the vehicle sound is looping, so, if I did not issue that command, the vehicle sound with the old volume would keep playing over the vehicle sound with the new volume. This does not happen when you shoot because the sound of the shot does not loop and is meant to be played on top of other sounds, not to replace them. For this, whenever you shoot, the command to stop every sound is *not* issued.
As I wrote in the previous post, I had asked in the Irrklang forum how to change the volume of a sound while it was played (as opposed to stop every sound and play the sound again with the new volume) but I had not received any answer.

I just wanted to also ask about the altitude markers on the left and right of the screen. In Damocles and Mercenary III the left shows the overall atmosphere of the planet and the right shows how high you are in relation to the ground.

Um... no. In Damocles and Mercenary III, the markers on the left and right side of the GUI are, respectively, the temperature and pressure markers. The temperature marker goes up during the local day and down during the local night, while the pressure marker goes up to a maximum as you descend while flying over the surface of a planet, and goes down if you ascend. They both go to zero in space. This is exactly the way they behave in TADPSID, it's just the scale that's different (they are 48 pixels high in the original games; their heights in TADPSID vary with the screen resolution, but they are proportional to 36 pixels at 640x480.

I just wanted to say the number 5 in the elevator on Clotho is upside down

It's actually not. The numbers come from an LCD font that I downloaded, and that is what the number 5 looks like in that font. This might be subjective, but I just tried to rotate 180 degrees the image of the number 5, to see if it really was upside down, and it looks worse. I'm leaving it the way it is.

and I also noticed the textures underneath the Eagle are backwards. I gather you are not supposed to see them anyway?

Yes, they are backwards because I textured it with a simple planar map, so you are seeing them from the correct angle (above) and the wrong angle (below). And yes, you're not supposed to see them anyway. I might modify the way it looks from below, but it's not a priority.

I'm sure in time you will make Mercenary IV just how everyone imagined

I'm not sure what you mean here. Do you mean that everyone imagines that I will make Mercenary IV, or do you mean that I will make Mercenary IV the way everyone imagined it to be?
In the first case, I hope I will be able to. In the second case, I can only say not to get too attached to something you imagine, because my vision for it might not match what you imagine. Of course, I will stay true to the outline that has been public domain since 1998 (the planet Marduk entering the Gamma system, your task being to stop the Nephilim - its inhabitants - from invading the system). You can be sure that Mercenary IV will be based on that premise. But the details of the plot that I wrote in 2010 in the never publically released game bible might not necessarily match what you imagine. For example, details of the solutions, or the way I tried to tie some loose ends of the previous games together.