I've heard a number of second-hand reports of sound quality issues,
but just wanted to convey some practical personal experience:

Try going to the vertical ellipsis (⋮) menu  in the bottom-right
corner of the Jitsi-Meet window, and select the "Manage video
quality" option, then select "Low bandwidth" mode. This will disable
video streams from all participants for you. You may have some luck
with its "Low definition" mode, but for me I needed to just dispense
with video entirely on the limited capacity system I was using.

A short explanation is that the video codecs used by Jitsi-Meet
benefit a lot from hardware acceleration. Many of us running open
source operating systems may lack built-in support for these, and
older/lower-end hardware can simply be overtaxed by it. When you run
out of available CPU cycles to process the audio streams, they start
to cut in and out. Closing other CPU-intensive applications can also
help, or moving to a dedicated machine if you have the luxury. As
mentioned elsewhere, modern WebKit-based browsers like Chromium do
somewhat better at handling this, so switching to one of those for
your conference window might help too. Further, the Jitsi-Meet
mobile app is reported to get good performance on some smartphones.
--
Jeremy Stanley