I've never used Sonar before, but would you have to set the MOTU up in some device setup, or something similar?

Sonar could be working off your internal sound card and not even looking for the MOTU.

I had this issue recently.
You have to set the options for the soundcard and options in like two other places. Took me four hours to get Sonar to record in 48/24.

Use the help function, in the search box type "24 bit".

If you can't figure it do tell, I may go and look.

My guess would be to go into Cuemix or whatever the control panel is for the MOTU and set it up as 24/48 then go into Sonar and load the MOTU ASIO driver as your sound device. It should automatically load at the samplerate that you set up. Sonar will likely run a diagnostic at this point to make sure that all is well.

As has already been suggested, you MUST tell your software that you want to use the MOTU as the soundcard.

I would recommend setting up a machine profile that disables your onboard soundcard for when you want to record and then re-enables when you want to do other stuff (games, etc.)

Just out of curiosity, why 48kHz? Are you doing video? If not and you're only doing audio, you're better off staying with 44.1 (albeit at 24 Bits). The resolution lost during sample rate conversion back to 44.1 (for CD) is worse greater than the resolution gained by bumping the sample rate up by >10%. If you want to explore higher sample rates, consider 88.2kHz.

Cheers-
Jeremy

Assumption: 88.2 goes to 44.1 better than 48?