the original instructions are quite simple , so I don't know whats wrong , but here , even simpler instructions .

download the attachment to your pc , plug the n900 , choose mass storage mode , extract the archive to your n900 .
there should be a new folder caller AudioBooster , inside it there should be two scripts and two folders .

disconnect your n900 from PC
on your n900 open xtreminal and paste the following :

sudo su
cd /home/user/MyDocs/AudioBooster/
sh ./AudioBoost.sh

make sure rootsh or sudser is installed .
---thats all