“Automatically add sound effects to this video” sounds like it should mean the tool actually watches your footage and places sounds to match it. Most AI sound-effects tools don’t do that — they generate audio from a text description, with no idea what’s actually on screen or when things happen. We tested one of each on the same silent clip to see how much that difference actually matters.
Two different approaches
Most AI sound-effects tools are text-to-audio: you describe the sound you want, and it generates a standalone clip with no awareness of a video’s timing or content. A smaller category is genuinely video-conditioned — the model watches the footage and places sounds automatically, without being told exactly where.
We took one silent 5-second clip and ran it through both approaches: Sonilo, which watches the video directly, and ElevenLabs’ Sound Effects model, given only a written description of the scene — it never saw the footage.
What this actually buys you
The video-conditioned result is synced directly to that specific clip — it’s built around this footage’s actual timing. The text-only result is a standalone sound design that has to be manually placed and timed against your footage afterward, the same manual work narration needs before it can be mixed into a video. Neither is strictly “better audio” — the real difference is where the work happens: automatically, at generation time, or manually, by you, afterward.
The practical takeaway
If you’re adding sound to a single finished clip and don’t mind manually timing it, a text-only tool works fine and gives you more standalone control over the sound itself. If you’re processing a lot of clips, or want sound that’s actually synced to visible motion without hand-placing it every time, a video-conditioned tool is doing meaningfully different (and more automatable) work — worth checking whether the tool you’re using actually looks at your footage before assuming “automatic” means what you think it means.
The source video above was generated fresh for this piece on fal.ai, the same platform behind our piece on combining video clips and our voice-quality piece.