Are you sure that you are replacing a photo with one of the same exact duration? Because if you are then an independent sound clip (one that is not attached to a clip above it) should not in any way be disrupted. Check your iMovie/Preferences and make sure that the Photos duration that is shown is exactly the same as the other photos in your project. This is because if you add a replacement photo it will come in at the duration that is shown in the preferences settings, that might be different than the photo that it is replacing. If that's your situation you need to exit out of the project, change your preferences, and reopen. Or, you can just drag the edge of the replacement photo to adjust it to the duration of the photo that it replaced.
When you replace a photo in the timeline, you drag the replacement photo directly on top of the photo to be replaced. You will then get a drop down menu with a "Replace" option to click on.
If you are still having issues, try deleting preferences. To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
Another thing that sometimes works is to place your photos into a newly created project.
From a work flow standpoint, it is often easier to place your videos and photos in the timeline first, and then add the audio. Or, you can ignore the audio, place all of your images, and then sync the whole project.
-- Rich