I see two - actually three - options here, from the top of my head.
* Display (static) image/ads in welcome text. Setting and updating it would be a manual step, thus would require ad management and adjustment.
* Display ad image/text in the welcome text which is set via a script (over Ice, Mumbles RPC protocoll).
* Regularly send an ad message to connected users - via a script (over Ice).
Using a script may not have the comfort of just embedding adsense HTML+JS code, but would allow the ads to come from a database. That way global management across servers would be possible, in contrast to the manual setting of ads, which would require management of individual servers.
: Your Mumble Web Interface in PHP