Firstly, using the browser on the sound manager will just play the audio locally.
Right now, as the sound feature is implemented. you can't technically broadcast an audio stream into other clients. What you can do is make sound files available by either making sure they have a local copy in the same directory location on their machines, or putting the sound asset in an accessible location, like a cloud hosting service, or if playing on a LAN, a shared device that everyone can access . You can then use the macro functions provided (playSound or playStream, these will pull up on the macro editor), to tell the clients to start streaming audio from that source. These functions accept string representations of directory paths, or the file:/ protocol, to resolve where the audio asset is.
The initial playback of an asset, if none of the clients have a local copy, will take some time to play. To ensure better future playbacks, Mote will make a copy of the asset and put it in the data folder, storing information about it in a database. The next playback will be nearly instantaneous, YMMV depending on the machine.
Online locations that work are Dropbox, and Google Drive, but you have to make sure that the links you broadcast, are the direct links to assets. So, in principle, any hosting service that provides direct linkage to hosted files, are usable. There are a lot of articles online on how to make direct links, and online tools that can do auto conversion, even in bulk.
Soundcloud integration is slated for Mote-X, which will make it even easier to stream audio to everyone.