If the arrival was programmed into the FMS, and the last arrival segment is a heading, as is often the case, the A320 will fly that heading to infinity. If the last segment ends at a fix, the A320 will continue present heading.
The cabin to flight deck interphone call signal would have awakened Rip Van Winkel, it is that loud.
Domestic operations often don't activate selcall as it is not used. ACARS is the primary company comm method.
Only this week, on a line check no less, we missed a frequency change and received an ACARS message from dispatch to the effect, "ATC wants you on 123.57." This is not uncommon at all, and wasn't even critiqued after the check.
Finally, in the expanse of Minneapolis, Salt Lake, and Denver Centers, through which the flight flew, you can go 20 minutes or more without hearing anything on the radio.
Finally, despite all the reports and speculation, we don't know what happened exactly, and I think it's premature to fix cause and blame with any degree of precision.