Okay. What is making the noise, a software synthesizer driven by the MIDI in track 1, or a hardware synth, such as in a keyboard ?
If a software synth, select both the MIDI track and the soft synth track by holding down the Ctrl key while clicking on the track number.
Then you can Bounce To Track(s) and choose which track for the bounced audio.
If a hardware synthesizer in a MIDI keyboard, you will have to connect audio cables from the keyboard to your audio interface's audio input. Select the corresponding input in your audio track, arm the track and click the Record button, or press 'R' on the computer keyboard.
Once that is done, you can Archive the original track if you want to, which will remove the track from processing.
This is about the most basic operation for any DAW. You might want to spend a little time with the manual.
Please pardon any typos and I can't post any images from Sonar, as I am on my phone .