After upgrading firmware to 2.0/2.0.1/2.0.2 and using it for a while, i found that some of apps that using audio output does not affected if we decrease/increase audio volume using music player volume slider.
So, i began jailbreak my device and googling for audio tweaks. Then i found Volume Booster for 2.0.1 and install it. But i think its only boost volume output for music player a bit, but not global, so other apps still not having effect.
Then i found this file (com.apple.celestial.plist) on my iPod Touch. Its in binary but if you want to know whats in it you can use some property list editor apps found out there, or simply use this http://126.96.36.199/~khchung/cgi-bin/plutil.cgi
It contains XML like this,
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
As you can see there is volumeLimits value is 1, and volumeMultiplier is 1.032174825668335.
So i try increase those value to 1.5 for volumeLimits and volumeMultiplier then reboot it.
Now others apps should have louder sounds when it plays.
To reset it, use Settings -> Music -> Volume Limit (slide it to whatever fits you)