Could an app make a sound after I thought I’d closed it?

I’d start with the phone, not with a conclusion about what caused it. Check which apps were recently active, force-close the app, restart the phone, and look at its microphone and background activity permissions. If you uninstall it, see whether the sound happens again. The exact settings depend on the phone, and permission details can’t prove what happened earlier, but they’re useful things to rule out.

A voice from an app can feel very specific, but hearing it once doesn’t tell you whether the app was actually running. There could be another audio source, or the app’s output could simply be easy to interpret as meaningful. I’d note exactly what you heard and what else was playing, then avoid treating the sound alone as evidence of a paranormal cause.

If using the app is making you anxious, deleting it and taking a break is a perfectly reasonable choice. For the journal, keep observations separate from explanations, such as “heard a voice while a video was playing” versus “the app was speaking in the background.” That lets you take your reaction seriously without locking in an answer. If you keep hearing voices when no app or audio is involved, consider talking with someone you trust or a health professional.