First, always extend the regions slightly longer than you expect the sound will play for, unless you want to cut the sound off at an exact point. This allows the whole sound to be played, even at a higher reading speed.
Second, review your mix at a reasonable reading speed - around 300 words per minute is pretty good. If you mix at a slow reading speed and then increase it, sounds may cut off or end abruptly. You can adjust the reading speed at the bottom of the page when you are editing sound by clicking on the "Sound" tab, or by adjusting it in the reader.
Third, try finding a composer you like and use various pieces from them. Each composer has a unique style that would make it feel cohesive. If a particular track has a composer, his/her name will be listed under the name of the track in the Track Selection tool. You can search by a composer's name to bring up all the tracks they have in any given category.
If you cant find the right composer, keep searching for other tracks with a similar vibe or feel.
Fourth, try to keep your mixes at an average volume. This is just a safety net so you don't distract the reader from the story. Having a short sound that is higher to emphasize a point is fine, but having every sound play at high volume can be distracting to the reader. You can adjust the volume level for each sound you add.
Fifth, almost always stretch music and ambient sounds longer than you would expect, even into the next scene. Increase the fade out time to be longer if need be to make the transition smoother. This sounds better than silence between scenes. Fading in effects is a good way to make them appear smoothly without taking you by surprise. For sound effects, we usually recommend adding a short fade (1 - 2 seconds) just to ensure the effect doesn't cut off abruptly. If the effect you added seems to start to quickly, increase the fade in time to smooth the transition. Keep in mind that this works well for longer sound effects like footsteps, but not for short effects like gun blasts. We generally like longer fade in/outs for ambiences and music. When you are moving from scene to scene and changing audio, overlapping the sounds with long fades works well.