Hi, HanCB1993,
When the slow-mo box is checked iMovie will apply slow motion to clips (lower the frame rate) added to the project that are a faster frame rate than 30 fps. When you uncheck "apply slow motion automatically" that will set your default for future projects, although it may not apply to your present project. In your present project, try doing an Edit/Select All, Edit/Copy, of your media in the project timeline, and then Edit/Paste it into the timeline of a project created after you changed your preference default. See if that works.
Another way would be to slightly increase the frame rate of the added clip that is being slowed down. You stated that manual syncing is not accurate enough for your music needs. I'll just mention it here for the sake of completeness and possibly for future situations. You would control click on the slo-mo clip to get a drop down menu where you can select "Show Speed Editor". When you click on "Show Speed Editor" that will reveal a speed adjustment handle in the upper right hand corner of the clip. Slide the handle to the left to increase the frame rate and to the right to decrease the frame rate. It is much easier to do the procedure if you use the slider located above and to the right of the timeline to expand out the time line. You can also use the speed adjustment controls that display when you select the clip and click on the loud speaker icon in the tool bar in the upper right of your screen.
-- Rich