I don’t know a lot about QSynth, but some pointers that may help:
(Update: Some information is updated in the next 2 posts, so read them too before attempting this)
Before you start, open a Terminal window and enter:
pacmd list-sinks
Look for the “sample spec:” line in the output. It will look something like:
sample spec: s16le 2ch 48000Hz
That tells you what sample rate PulseAudio is using. Use that sample rate for both QSynth and Audacity.
As a general tip, if you have any pavucontrol windows open, close them before making changes to your set-up. Open pavucontrol when you need it, then close the window.
Open QSynth. You may get an error message about being unable to start the audio engine - that’s OK, we haven’t set it up yet.
Click the “Setup” button.
On the “Audio” tab, set the “Audio Driver” to “pulseaudio” and the “sample format”, “number of channels” and “sample rate” to match PulseAudio (as previously shown in the Terminal). Everything else can probably be left at default for now (search on line for more details if required).
Click the “OK” button.
You may be prompted to restart the audio engine, if not, you can press the “Restart” button anyway and you will be asked to confirm.
That should be it for QSynth. Check if it is working.
When QSynth receives MIDI data you should see a green “light” in the lower left corner of the interface - that is how you can see if it is receiving MIDI.
Open Audacity. If it’s already open, exit and re-launch Audacity.
In the device toolbar, set the recording input to “default” or “pulse” (“pulse” is usually the “default”).
Set the “Project Rate” (lower left corner) to match the PulseAudio sample rate. You can set this as the default by going to “Edit > Preferences > Quality”)
Click on the recording meter to start monitoring. (there is also a drop-down menu, indicated by a small black triangle near the microphone icon).
Open PulseAudio Volume Control.
You should see “Fluid Synth” listed in the playback tab.
You should see Audacity listed in the recording tab.
If Audacity is not listed in the recording tab, click the record button in Audacity and it should then appear (it’s not always easy to see if the meters in Audacity are active or not).
Set the recording source for Audacity (in pavucontrol) to the Monitor option for your sound card. Note that if you have more than one sound card, there will be more than one “Monitor” - you need to select the one that “Fluid” is playing through.
Audacity should now be set to record “Fluid Synth” (QSynth is a GUI front end for Fluid Synth).
If all is well, close pavucontrol, and start recording.