This is a method to increase the sound for a text alert.
First, download Audacity from the following link:
http://audacity.sourceforge.net/download/Choose version 1.2
You will need to LAME mp3 encoder to export as an mp3:
http://audacity.sourceforge.net/help/faq?s=install&item=lame-mp3Your text tone must be in an mp3 format.
First, take the text alert file and drag it into Audacity.

It should look like this:

Next click at the very beginning of the sound clip. To do this easily, click anywhere in the sound area (with the purple sound waves) and press the Home key on your keyboard.
Now click the Generate menu and select Silence:

Change it from 30 seconds to something like 3 seconds.

Then click Generate Silence.
This will be the result:

Now click on File > Export as MP3:

If this is your first time using Audacity, then you will need to browse for the lame_enc.dll file. From the LAME mp3 guide:
The first time you use the "Export as MP3" command, Audacity will ask for the location of "lame_enc.dll". Navigate to "C:\Program Files\Lame for Audacity", select "lame_enc.dll", then click "Open" and "OK".
It will then ask where you want to save your mp3 file. I usually choose to rename the file to something like "Message 1 withSilence" instead of overwriting the file.
Finally, transfer the new file to your phone and set it as your text message alert tone.
If you have any problems or need clarification on anything, please post a reply.