Comment 18 for bug 442748

As suggested in #17, I've configured my system as follows per https://help.ubuntu.com/community/UbuntuStudioPreparation

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Real-Time Support
After you've got the kernel you still need to set up real-time access for your applications.
All you have to do for this is give your audio group permissions to access the rtprio, nice, and memlock limits. To do this, you just need to run these commands, which will add some lines to the file /etc/security/limits.conf:
 sudo su -c 'echo @audio - rtprio 99 >> /etc/security/limits.conf'
 sudo su -c 'echo @audio - nice -10 >> /etc/security/limits.conf'
 sudo su -c 'echo @audio - memlock unlimited >> /etc/security/limits.conf'
(I replaced "unlimited" to 1024 because my machine has 2GB of RAM)
These value are suggested by http://jackaudio.org/faq. The memlock line determines how much of your memory can be locked by audio processes. Some recommend setting this as half of your total memory (RAM, in KB). See Florian Paul Schmidt's page.
If you use a Firewire sound card, you already done the step described here: in Hardy and Karmic beta, this group is already created at installation. You just have to do:
 sudo adduser <username> audio
Restart Ubuntu and it is ok.
If you experience freezes after rebooting Jaunty Jackalope, try removing the "memlock unlimited" line or changing it to a different value. Always keep a -generic kernel on your Grub list. If for some reason you can't boot with the -rt kernel, it will be very useful.
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Thanks Daniel. I'll let you know how it works for me.