Does your stereo completely switch off (i.e. no power at all) as if you were using the on/off knob?
I ask because my rcd510 has recently developed an issue with loss of sound on ignition startup. Stereo comes on as normal when ignition key initially inserted but 2/3 secs after engine is turned over, stereo sound is halted, as if it's been muted (it's not), though the stereo remains switched on. The remedy is to actually mute it using steering wheel button then unmute it, then the sound returns. At first I thought it was an iPod fault but it also happens when the radio is playing. It happens every time car is started up and is becoming pretty annoying.