All right... The first thing is to get all your sound elements into a single Audacity window. Start by File->Open for the first element. Then use File->Import->Audio to add more. If they are in stereo, split them to mono by clicking the triangle at the start of each track. Since they most probably come from a single source, you can then delete the other channel. I couldn't find any sound files through your link, so my instructions are more general.
So, let's say you want to make a mix of several P51 sounds to be used in AH. Import, split and delete until you have a bunch of mono tracks at hand. You can listen to them all at the same time or mute one or more of them even while listening. If you want tracks start at different times, use Tracks->Align Tracks->Align with Cursor to change the starting point of each track. Don't bother about different lengths because 10 seconds is all you'd need.
When you have your selection ready, the Effect menu is what you'd mostly need. Use Amplify to boost or diminish desired channels. Ctrl-z (undo) is your best friend! When you're happy with the result, choose all tracks and use Amplify again with a negative value to reduce the overall sound level. The tracks add up when mixed and we don't want any distortion or clipping, do we... Then go to File->Export and save it as a wav file which will be the raw material for your sound clip. You can import the clip you just created to see if the curve fits nicely into the track window. Then save and close your project.
Now open the wav file you've just created. Listen through it and look at the time line to find the best sounding part. Select that part with your mouse, adding some extra at both ends and copy it (Edit->Copy or Ctrl-C). In the Tracks menu choose Add New->Audio Track and paste your clip there. Select that clip. Use the Zoom button to magnify the sound curve until you clearly can see the ups and downs, preferably as a continuing pattern. Select about 10 seconds starting and ending at a point where the curve passes the zero line, then Edit->Find Zero Crossings (keyboard z). Now your selection should start and end at a zero crossing. Take a closer look (zoom in): Would the curve make a perfect loop? If the curve starts rising from the start, it should end by rising from the negative bottom to zero. You can move the endpoints of your selection with your mouse and redo the zero crossing search until you've found a loopable pattern. When you've found it, copy-paste it to a new audio track. Transport->Loop Play will tell you how it would sound in AH. If you can't hear where the loop restarts, you've succeeded.
Now is the time to fine tune your clip. If you want to boost lower frequencies or add echoes or other gimmicks, do it now. Notice that many effects add volume, so keep an eye on the sound curve. Reduce the volume if necessary. After you've finished, use Effect->Normalize to amplify your sound right below clipping/distortion. File->Export your work as a Wav file with the correct plane name and place it into your Hitech Creations\Aces High\Sounds folder.
