fix(browser): stop creating throwaway AudioContext in getSampleRate

[SergioChan]

Summary

• change Speakers.getSampleRate() to return 44.1kHz by default unless playback has already started
• reuse this.audioCtx.sampleRate when an active audio context exists instead of creating and closing a temporary context
• add regression tests confirming getSampleRate() no longer constructs AudioContext and still reports the active context rate

Testing

• node --test test/speakers.spec.js

Related

Fixes #613