Interesting question. I looked into the issue and it's doable, but converting all files will take some time.
The main problem was identifying the audio format. All audio files are stored raw, which means the files don't have a header with any information about it's format. The good thing, raw files are supposed to be uncompressed which shrinks the options. After some try and error, I came to the conclusion, that the voice files are 16bit PCM mono with a sample rate of 22050 Hz.

Audacity is capable of opening and saving raw PCM and offers a normalizing feature (and it's free open source software!).
0. Make a backup of all sound files, just in case something goes wrong.
1. Start Audacity
2. Select File => Import => Raw Data...
3. Choose one of the game's raw voice files (I tested it with \Airline Tycoon Deluxe\VOICE\Bo\700.raw It's the first sentence the Boss says in a Freegame)
4. Use the following settings:
Encoding: Signed 16 bit PCM
Byte order: Little-endian
Channels: 1 Channel (Mono)
Start offset: 0
Amount to import: 100
Sample rate: 22050
5. Alter the clip to your liking; For a simple normalization, select Effect => Normalize... => OK
6. Select File => Export Audio...
7. Choose file type "Other uncompressed files"
8. Click on Options... and set Header to "RAW (header-less)" and Encoding to "Signed 16 bit PCM"
9. Save the file and replace the original version.

As I've already said, it'll take a while to do this with all voice files. What might speed up the process is importing several files, running Normalize once for all of them and then use File => Export Multiple... with the same settings mentioned above, thus only the import has to be done for every single file separately.