I have recorded some hold music from a telephone call using the headphone out jack from my smartphone, running to the line-in of my Focusrite audio interface and from there to Audacity running on the computer.
The sound quality is pretty low, with some low-volume crackles every now and then and a volume way too low.
I post-processed the recording in Audacity by normalizing the audio and running the noise reduction filter, but I hope there is something else that can be done.
Is there any way I can improve the quality of the source material (i.e. how can I record the music from a telephone call better)?
How should I post-process the recording to maximize the quality?