This article explains how to add sounds to your TikTok videos on iOS and Android. The TikTok app comes with a selection of sounds, but you can also record your own; third-party video editors also work.
How to Make Your Own Sound on TikTok | Add Your Own Music to TikTok
The app has a built-in voiceover feature that lets you import a new sound that wasn’t in the original video. Follow these steps to record audio over your TikTok video.
Open the TikTok app and select the plus sign icon at the bottom center.
Record your video through the app by clicking the red record button. You can also upload a video (or multiple videos) from your device by selecting Upload and choosing the video(s).