Firstly, you need to download the tone to your PC from Free ringtones, themes, wallpapers, games. Millions of downloads for your mobile phone | Zedge
Then, go to Google and search for Audacity, it should be one of the first links that come up.
Install the program to your computer and run the program.
Once the program is running, you want to import your downloaded ringtone. Select all the tone, click on Effects and then click Amplify. Then just keep playing with the tone until you have it as loud as you like, click ok.
Once Audacity has edited your tone, click on File, then Export tone as mp3. This will save your ringtone as an mp3 file. Then, you can either email yourself the tone, or you can connect the BB to your computer via USB cable & copy the tone to your ringtone folder on the memory card.
Hope that helps. If you have any other questions, feel free to PM me with them and I'll do my best to help.