Help please - Sony HU, why 1/2 sec attenuated sound?

My setup: Gen 3 Prius II, Sony 660BT head unit. "Everything" is wired up. Everything works perfectly, except for one significant annoyance:

At random the sound level smoothly and briefly attenuates to a very low level, then back to normal.

• The whole event lasts less than a second
• Happens at random, from 5 seconds apart to 45+ minutes apart
• The dropout is smooth, not a "cut off"
• Happens both on radio and iPod. (At first I thought it was just weak radio stations)
• Crutchfield had me do two tests:
  • Ensure the HU is grounded only through the wiring harness
  • Try providing power directly from the battery (that was fun... big cables from the trunk up front )
  • Nothing was found wrong
• They sent me a replacement unit. Same exact symptom.

A friend suggested perhaps the Sony speed-in wire is attenuating the volume, and doing it badly. I'm pretty sure I wired it correctly, but who knows.


QUESTIONS

• Have you ever seen this type of thing, on any head unit?
• Any ideas on how to further diagnose this problem?
• Any ideas on what the answer might be?
• Any ideas on the purpose of the Speed-In wire, and the impact of leaving it disconnected?

Thanks MUCH!

 

GREAT guess, very close to the correct answer. Thanks!

I don't know how it got to this state, and I'm recommending some changes to Sony, but here's what was happening:

• First, there is only one volume control when it comes to the various navigation signals, directions, etc. In my case, having the volume high or even completely muted made no difference.
• There are multiple ways that certain nearby POI's can produce a signal/warning. One is in "Manage POI's" where one can select any/all POI categories to produce a warning. The other is in the Advanced settings menu, in the Safety warnings, where one can ask to be warned when near schools, etc.
• Here's where it gets strange. The "active" warning signal includes a silence option. Yes, you can be warned that you are near a POI, with a silent warning.
• Even more strange: this silent warning "sound" remains active even if the associated POI category display has been disabled.
• Bottom line: any time I drove within 1/4 mile of any kind of educational institution (even obscure things like a Radio Shack Training office two blocks over), my sound would briefly disappear... with no other indication of why!

Implications:

• Sony should remove the "silent" warning signal
• Users should verify the warning signal choice behind any configured signals.
• Probably best to only enable warnings for POI's that are visible.

 

Even more confusing when near invisible POIs. That's why it seemed random and confused me, and both Crutchfield and Sony tech support.

We're all quite relieved... and they were very glad to receive new info for their support databases