I frequently use the tempo ratio buttons to adjust tempos while I'm recording. To make them functional, the clock must be set to "internal." While recording, the clock frequently resets itself to "Audio" even though I've done nothing (that I'm aware of) to cause the clock to reset. Has anyone else seen this behavior and, if so, arrived at a solution?

As you probably know, the 'internal' setting is only appropriate for pure MIDI projects driving direct-monitored external hardware.
 
If there's any audio-based content in the project at all, the clock will default to Audio. This includes the audio metronome, soft synths, and input monitoring an empty audio track.
 
 

Sonar can only really keep the clock at "internal" when there is no audio. No audio tracks, no VSTis. If there is audio input or output, Sonar is forced to reckon time by the rate of samples as clocked by the audio hardware. As you have found, Sonar only offers tempo ratios for "internal" clocking, so no audio stretching.

Thanks for the suggestions; however, this are no audio tracks, soft synths or any other audio components. The project is pure MIDI.

Don't know, then. I have not tried to use 'Internal' for some time, though in the past it always persisted so long as I studiously avoided enabling any audio-based features. Possibly it's related to a reset that occurs when entering/exiting Preferences...?
 
 

Well I was unable to get the clock to stick to internal at all. I deleted all buses and unchecked all audio devices. Playback and record timing masters show none or empty. With just one pure midi track, when I press play, the audio engine indicator does not light up, but the project clock source switches to audio. (Manchester)

I just tried, and could not reproduce an issue.
 
- Start a new project from the BLANK template using File > New.
- Verify clock is defaulted to 'Internal', and Metronome is set to use a MIDI note.
- Insert a MIDI track and arm it.
- Record MIDI from a keyboard synth.
- Play back the MIDI driving the direct-monitored keyboard synth.
- Save, Close, Re-open, and Start/Stop playback again.
 
After doing all that, I verified the clock was still set to 'Internal'.
 
 

How strange, my main difference was I started with the 16 midi channel template. It routinely changes clock back to audio. When starting with a blank project adding one midi channel as you (brundlefly) tried, the clock stays internal.
 
Edit: now I see metronome for the midi template is audio. (!)

bvideo
Edit: now I see metronome for the midi template is audio. (!)

 
I figured.    You wouldn't expect that from a MIDI template, but the creator probably didn't think about it.
 
OP hasn't really ever confirmed he checked the metronome in his project.

In response: I always use a MIDI click for the metronome, not an audio one. Also FWIW, the file was created by adding individual tracks, not from any template.
I have disabled the audio interface (Tascam MKII), but am wondering if the problem is caused by the Kronos USB interface that provides both MIDI and audio, even though the Kronos audio is also not being used.
To repeat my original problem: the clock resets itself to audio while working on a project, even though the project has been using the internal clock. It's not a major task to change the clock, but I cannot understand why this is happening.