Two different operations share the word merge
Mixing tracks together (like a voiceover layered over background music, playing simultaneously) and joining tracks sequentially (like combining separate song files into one continuous playlist file) are fundamentally different operations that both get called merging in everyday language — mixing requires balancing relative volumes so neither track drowns out the other, while sequential joining is purely about order and transition, with no simultaneous playback involved at all.
Getting clear on which one you actually need before starting saves a lot of confusion, since the settings and considerations for each are quite different.
Balancing levels when layering tracks
When mixing a voiceover over music, the most common mistake is leaving both tracks at their original recorded volume — background music recorded or sourced at full volume will usually overpower speech, making it hard to understand. Lowering the background track significantly (often by well more than half) while keeping the voice at a clear, comfortable level is the standard approach, sometimes called “ducking” when it's done dynamically to only quiet the music while someone is actually speaking.
Length mismatches need a decision, not an accident
When layering two tracks of different lengths, you need to decide what happens at the boundary where the shorter one ends — should the longer track continue alone, should it fade out at that point, or should the shorter one loop to fill the remaining time? Leaving this to chance often produces an awkward abrupt silence or an unintentional jump, so it's worth deciding deliberately rather than accepting whatever the tool happens to do by default.
Clipping risk doubles when you're combining, not just boosting
Layering two full-volume tracks together can push the combined signal past 0dB and cause clipping, even if neither track alone was clipping — this is the same ceiling that applies to volume boosting, but it's easier to overlook when combining tracks because each one individually looked fine. Checking the combined output's peak level after merging, not just each source's level beforehand, catches this before it becomes an audible problem.