I've got a couple projects in mind where I will be needing the Arduino to bridge a connection and I'm wondering when to use a transistor to do so or a relay.
Application one would be using the Arduino to selectively ground a music instrument signal. Sort of as a binary on/off kill switch. No audio would be passing through the component, but would (hopefully) be doing it very quickly (almost ringmod like on/off muting).
The second application would be using the Arduino to control a normal 'stomp' switch (momentary style BOSS switch). I'd like to have the Arduino control when the effect turns on and off. Again, no actual audio would pass through the switch, but it would do it quickly.
In general I'm not sure of when one would use a relay to accomplish switching in audio based applications. From my understanding relays can handle high voltages and such, but this isn't really needed in audio based applications.