The dialog will appear if you switch track patch mode from Mono to Stereo and any audio track has pan or volume adjustment.

Replication steps:

1) Right-click on audio track patch then disable “Audio Source Channel (Stereo)” mode.
If the option is checked, click the option to uncheck.

2)      Place a clip onto the timeline. Audio will be placed to 1ch independent track.

3)      Adjust either volume or pan in any audio track.

4)      Right-click on audio track patch then enable  “Audio Source Channel (Stereo)” mode.

5)      The dialog will appear.

EDIUS audio track can place up to 2ch audio stream per one audio track if you enable Stereo mode for source channel.
However, volume or panning rubber band is single and you cannot set them for each track independently if stereo mode enabled.
Volume or pan can be modified independently if mono mode and you did it in step 3 in above. EDIUS asks you to apply volume or panning adjustment of odd channels to even channels by appearing the dialog. This is for the clip mounted in player window:

Yes button: EDIUS mounts timeline clip to player with all reset of volume and panning adjustment.
Adjustment in timeline won’t be lost.

No button: EDIUS mounts timeline clip to player with keeping volume/panning adjustment those have been done in odd channels.

EDIUS 9.3 Requirements to create a proxy

Video FrameRate: 23.98, 24, 25, 29.97, 30, 50, 59.94, 60 (fps)
Audio SamplingRate: 48000, 44100, 32000 (Hz)
No Alpha 

If video clip don't meet the requirements, EDIUS 9.3 can't create a Proxy.
Additional, if you change the frame rate of high res clip your proxy file in the timeline could get offline.