Soundwave will create a macro to summon a random companion from a preset list. The macro will take up one space in each characters personal macros and the list is saved per character between sessions.
You can use /swave or /soundwave to open the window (as seen on the right) to select your companions. The list will be saved when you close the window. Add or remove a companion by clicking its icon. If it's desaturated, it's not selected. If you want to you can SHIFT-click a companion to summon it that particular one, but it won't be added to the list.
The macro updates whenever you summon a companion (or lose one for whatever reason). If you're currently in combat it will update as soon as you exit combat. There are some measures against getting the same companion twice in a row, but you can still get the same every other random if you're "unlucky".