The source isn't in the sid repo but Audacity is.
Edit: or maybe it is, anyway easy enough to download it:
$ apt-get install audacity -s
NOTE: This is only a simulation!
apt-get needs root privileges for real execution.
Keep also in mind that locking is deactivated,
so don't depend on the relevance to the real current situation!
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
audacity-data libportsmf0 libsbsms10 libvamp-hostsdk3
The following NEW packages will be installed:
audacity audacity-data libportsmf0 libsbsms10 libvamp-hostsdk3
0 upgraded, 5 newly installed, 0 to remove and 1 not upgraded.
$ apt-cache policy audacity
unstable/main i386 Packages
I have maybe 5 apps that weren't installed using apt-get/dpgk
Eight years of running sid and those are the only apps I have used that I didn't install with apt-get.
I may not use many of the repos in my Giant sources.list but there are a ton of 3rd party repos for Debian.
$ inxi -r
Repos: Active apt sources in file: /etc/apt/sources.list
deb http://ftp.us.debian.org/debian/ unstable main contrib non-free
deb-src http://ftp.us.debian.org/debian/ unstable main contrib non-free
deb http://ftp.us.debian.org/debian/ experimental main contrib non-free
deb http://www.deb-multimedia.org/ sid main non-free
deb http://dl.google.com/linux/chrome/deb/ stable main
deb http://moblock-deb.sourceforge.net/debian/ sid main