I use Audacity for (multitrack)recording my electric guitarguitar. I use the outputchannel from my Zoom multi-effect and an I-Rig HD2 guitar interface.
The first recording is ok but every time I make an overdubbing record the sound quality is getting worse. Using presets effects like delay etc makes it
even worse. Has anyone tips to improve the sound quality??
Does the 2nd track sound bad by itself or does it only sound bad when mixed?
It sounds bad (not as good as the first one) by itself. Mixing makes it even worser.
It sounds bad (not as good as the first one) by itself
That’s probably [u]dropouts[/u] which are related to multitasking interrupting the recording. That can be difficult to fix because the computer is always multitasking even if you’re only running one application. 
[u]Here[/u] are a few suggestions. If none of that helps there is a free online book about optimizing your computer for audio called [u]Glitch Free[/u].
…When you record the audio data streams into a buffer (memory like a holding tank or a long pipe) at a smooth-constant rate. Then when the operating system gets around to it the data is read from the buffer and written to the hard drive in a quick burst. If the buffer doesn’t get read in time you get buffer overflow and a glitch. This can happen even if you are not using 100% of your CPU time… Some application/process/driver just has to hog the system for a few milliseconds too long. There is also a playback buffer that works the opposite way - It gets filled in a quick burst and the sound/data comes out a smooth-constant rate. You get buffer underflow if the playback buffer is not re-filled in time.
Mixing makes it even worser.
Mixing is done by summation so you can get [u]clipping[/u] (distortion). That’s easy to fix -
If you are mixing two files you can reduce the volume by 50% (-6dB) before mixing.* Or you can export as 32-bit floating point WAV, which can go over 0dB without clipping. Then re-import and Amplify or Normalize to bring the levels down before exporting again to your desired format.
Audacity also uses floating point “internally” so Audacity itself won’t clip. But you’ll clip your DAC (digital-to-analog converter) if the file/data goes over 0dB and you play it a “full digital volume”.
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- Analog mixers and more-advanced mixing software has a volume control for each input/track plus a master volume control making it easier to control the overall volume.
the computer is always multitasking even if you’re only running one application.
Overdubbing stresses the machine. It has to manage multiple sound pathways perfectly and in real time. There is no machine “Hang on while I check something.” If you were just getting along OK with simple sound recording, overdubbing can push you over the edge.
There is one first-level thing you can try. Shift+Shutdown, wait, start. That performs a clean shutdown with no other baggage. Don’t let anything else start. Then disconnect any network drives or cloud storage. Those can cause troubles. See if it starts working.
But it is super common to get the backing track and the new performance jammed into one track. That’s what happens when you like to record Youtube or other internet content. The settings for that are exactly wrong for overdubbing.
Koz