My problem: I can record and listen to the recording in both loudspeakers as headphones but, when I record the next track disappears sound from the other tracks and I hear only the sound that is been recorded.
The program worked properly until two days ago.
What may have led to this happening now?
You didn’t say anything about what you were recording, but the settings for what you want are in Audacity > Edit > Preferences > Recording.
[X] Overdub is the setting that plays the existing tracks while you’re recording new ones. If you have multiple tracks, you can pick what you want to listen to with the SOLO and MUTE buttons to the left of each track.
[X] Playthrough is the setting that allows you to hear the existing, live track going down.
There is a problem that Playthrough is “one computer late.” You can’t as a rule listen to your own live musical performance that way because it’s going to be late or have an echo. Most people can’t perform by listening to Playthrough.