Audible indicator warnings can be fitted, see the M&P catalogue, also some Accumen alarms have this feature.  I had a hire bike with one and the longer the indicator was on, they louder they became, so if you were waiting at a busy junction it drove you nuts.  I was out with an advanced riding instructor and it was driving him bonkers as well    I must replace my damaged Hi-Viz (didnt help the taxi driver see me when he pulled out in front of me!).  I used to carry one under the seat for riding on dark unlit roads or motorways in bad weather condidtions.  Anyone know where I can get a replacement open mesh Hi-Viz as I cant seem to find one anywhere.                                                                                                                                                                             

Sportouring web site have lot of nice stuff including the TURNSAFE Audible Indicator Warning. But with this one after 10 flashes it starts beeping and also mutes the beep while the brake lights is on, £40 sound expensive to me though.