I don't know of a specific app, but my 9800's speakers are weaker than my friend's 9700's. I'm not sure if it would be a good idea to make the ringtones louder than the speakers can handle. They might start to blow out on you.
Thought someone found out that the Bold 9000's Speaker was used in the Bold 9700 and now in the 9800 Torch? I can't remember if it was a article, or a forum thread tho
I'm pretty sure no phone has ever had speakers as good as the 9000. The 9700's were definitely not as good.
The Torch speakers are not as good either but to me they sound ok. Ringtones are plenty loud for me. The OP may not be using the loudest settings and/or may have downloaded ringtones from somewhere with lower volume.
I'd suggest using the link in my signature for information on how to install OEM ringtones.