The fastest way to find the repository in which a package was published, is to search the ubuntu package database. For the package aac-enc we will find the following among entries for different releases:
- trusty (14.04LTS) (sound): Fraunhofer FDK AAC Codec Library - frontend binary [multiverse]
0.1.2-1: amd64 i386
The repository will be printed bold in square brackets ([multiverse]).
A personal package archive (PPA) is something different, as it allows everybody to upload any package there. Therefore there is an inherent risk to install packages with issues.
Searching for a PPA can be done from launchpad:
will tell us that the package aac-enc is part of the package fdk-aac. On clicking on that we will be directed to a download page for different releases. Further down there is a link to versions in untrusted archives:
This links to an unsorted list of personal package archives containing fdk-aac but we can not limit this search to a defined release.