Using window 8 and version 2.3.0.
When I record the guitar I stayed at about -12, same for the bass and drum. However when mixing into one track I go into the red zone. I tried to record to much lower level but it sounds so weak. Any suggestions?
When mixing tracks, the tracks are literally added together, so in most cases the mix will have a higher peak level than any of the individual tracks.
When recording, we generally recommend aiming for a maximum peak level of around -6 dB (half the track height). This is the case whether recording just one track or making a multi-track recording.
Obviously if we add together a lot of tracks that have a peak level of -6 dB, there’s a very high chance that the mix will clip, so to avoid that we need to reduce the level of each track so that the mix stays below 0 dB. A good way to do this is with the track’s “Gain” slider: https://manual.audacityteam.org/man/audio_tracks.html#gain
The track’s Gain slider does not actually change the track audio - it just changes how loudly it plays.
These sliders are equivalent to the vertical sliders in the “Mixer Board” https://manual.audacityteam.org/man/mixer_board.html
When using the Mixer Board (of track Gain sliders), keep an eye on the playback meter to ensure that the mix remains below 0 dB.
Before exporting the mix, make a backup copy of the project (“Save Project As…” with a different project name, or “Save Lossless Copy of Project…” are both good ways to make backups https://manual.audacityteam.org/man/file_menu_save_project.html)
Then select all of the tracks and “Mix Down to New Track”.
Then solo the new track so that all other tracks are muted (https://manual.audacityteam.org/man/audio_tracks.html#solo)
You can then amplify or normalize the mix track to ensure that the peak level is close to, but below 0 dB (I usually Normalize to -1 dB).
Note that commercial recordings generally use lots of compression and limiting to increase the loudness without exceeding 0 dB (see: https://en.wikipedia.org/wiki/Loudness_war)