Most development discussions on aRts take place on two mailing lists. This is the place to discuss new feature and implementation ideas or ask for help with problems.
The KDE Multimedia mailing list is for general KDE multimedia issues including aRts as well as the multimedia applications like Noatun and aKtion. You can subscribe from the web page at http://www.kde.org/mailinglists.html or send an email with the subject set to subscribe your-email-address to <email@example.com>. The list is also archived at http://lists.kde.org.
The aRts mailing list is for issues specific to aRts, including non-KDE use of aRts. To subscribe, send an email containing the message body subscribe your-email-address to <firstname.lastname@example.org>. The list is archived at http://space.twc.de/~stefan/arts-archive.