OK .... I won't go so far as to start talking about FSX file structure and ALIAS= data .... because I'm not familiar enough with it and will likely only put you crook.
I can only speak intelligently regarding FS2004.
However .... allow me to explain as follows though because what I do say (as follows) should be the same regardless of whatever FS version is being used ....
1. Obviously you've successfully downloaded and installed your B707-320C AIRCRAFT BASE PACK. You've obviously downloaded and installed a texture or 2 as well .... and all these are displaying/working fine within FSX .... correct ?
"IF SO" .... then now please carefully note the following ....
All our AIRCRAFT BASE PACKS contain the following primary data folders ....
MODEL
PANEL
SOUND2. In the case of your audio we need to focus upon the folder named "SOUND".
The default sound folder (which accompanies the downloaded AIRCRAFT BASE PACK folder) will contain only a SOUND.CFG file. When opened and read this CFG file will read as follows ....
[fltsim]
alias=B737_400\sound.... meaning that unless this folder is removed, or the ALIAS= data lines changed, that particular aircraft will always use the default MSFS B737-400 soundpack.
I think Dan (and all FSX users too) will agree with what I'm about to say next ....
"IF" what I've described above is precisely how the "SOUND" folder of your B707-320C AIRCRAFT BASE PACK appears .... then you can safely delete or remove this entire folder .... and replace it as follows ....
1. Go to the "BOEING SOUNDS" page of our DOWNLOADS section.
2. Select either the "P&W JT3D-3B" .... or .... "P&W JT3D-7" soundpacks and download either one of these. Most B707-320C aircraft were powered by the D-7 engine, but, a few were also powered by the 3B engine version. The tonal differences between both are very slight.
3. UNZIP the contents of the downloaded soundpack to a temporary folder.
4. Firstly .... go back to your B707-320C AIRCRAFT BASE PACK folder and delete the default "SOUND" folder there. This will then leave you with the following 2 primary folders ....
MODEL
PANEL.... along with whatever TEXTURE folder you have installed at this point.
Don't touch any of these other folders .... OK.
5. Go back to the UNZIPPED contents of your downloaded P&W soundpack .... identify the new folder named "SOUND" (it contains all the WAV files for this particular audio) .... and simply drag & drop this entire new "SOUND" folder into your B707-320C AIRCRAFT BASE PACK which will then also restore you back to the following 3 primary folders ....
MODEL
PANEL
SOUND.... along whatever TEXTURE folders you have installed.
Doing this should now give you high quality custom P&W fanjet sounds .... both during interior and exterior viewing modes. The aircraft can't possible then use any other audio.
"IF" this doesn't work (and there's no reason why it shouldn't) then something else has gotten stuffed-up much earlier during your installation process .... and in that case I'd rather leave you in the hands of our "Great Dane"
.... Dan The Man FSX Expert Extraordinaire !
You should be fine though in so far as following my above recommended instructions .... OK
FYI .... if you're also using the recommended HJG/CARTY B707 panels I urge you to read the engine startup tutorial associated with my B707 PANEL INSTALLATION & HANDLING NOTES thread .... on the "ABOUT HJG AIRCRAFT" forum page. Follow those procedures precisely and you can't possibly go wrong.
Mark C
AKL/NZ
.... Dan The Man FSX Expert Extraordinaire ! 
