This paper focuses the prototyping stage of the design cycle of ubiquitous music (ubimus) ecosystems. We present three case studies of prototype deployments for creative musical activities. The first case exemplifies a ubimus system for synchronous musical interaction using a hybrid Java-JavaScript development platform, mow3s-ecolab. The second case study makes use of the HTML5 Web Audio library to implement a loop-based sequencer. The third prototype - an HTML-controlled sine-wave oscillator - provides an example of using the Chromium open-source sand-boxing technology Portable Native Client (PNaCl) platform for audio programming on the web. This new approach involved porting the Csound language and audio engine to the PNaCl web technology. The Csound PNaCl environment provides programming tools for ubiquitous audio applications that go beyond the HTML5 Web Audio framework. The limitations and advantages of the three approaches proposed - the hybrid Java/- JavaScript environment, the HTML5 audio library and the Csound PNaCl infrastructure - are discussed in the context of rapid prototyping of ubimus ecosystems.