Bus bells? Hmmm... I have quite a number of observations and comments about them...
I know that both SBS 1688K and SBS 2888T have that afore-mentioned "Sentosa chime" for their door-closing alarm too, but the one for the front door of SBS 1688K has since been replaced. Interestingly, that is also the same sound for some of SMRT's Malaysia Dennis Lance's bell; that chime, I believe, is actually from a commercially available door bell (I once found it in a DIY shop), and it has two settable variations: "ding-dong" and "ding-dang-deng-dong". The original bus bells, I believe, also has two variations: "ding-dong" and the "Sentosa chime" (two series of quick "ding-dongs"). I used to run quite frequently into a DM3500 a few years back whose bus bell was set to use the "Sentosa chime" (it might have been corrected since), and sometimes I hear it too on route 39 buses when the driver switches on the engine while I'm waiting for it at Yishun Interchange.
As for those "perh" buzzers on SMRT buses, I suspect SMRT took them from those old, since scrapped Hinos and Nissans just before they were scrapped (along with (probably) the ez-Link card readers, the ticket validators, and whatever else that looked like they were reusable

)). Some of them sound seriously terrible, but I know there's one route 169 CAC MB O405 that sounds quite well; it's like "teah" rather than "peh" or "perh", and I find it fits into the ambience of the bus quite well. I could, however, prefer them to be installed on those Nissans though - adds nostalgia

.
Other random observations:
��• Yes, the box bendies' "Bus Stopping" sign is too faint and confusing, I have to agree. However, it's amusing that they use the same type of "ding-dong" bells as most of the SBS Transit buses

. At least one Malaysia Dennis Lance use that too

.
•• The SMRT Volgrens and the UK Lance's "Bus Stopping" sign is somewhat irritating - I don't really like how it keeps on flashing until the exit doors are opened and then closed. The "bee-boor" sound is also rather... toyish? Although the Volgren bendies sound a bit more professional, but rather botched-up.
• Anyone notice how some of those window sill bus bell buttons on older SBS Transit buses (which lights up) has burnt-out plastic? On some buses, the light bulbs inside sometimes don't work too. By the way, some B10M MKIIIs, after refurbishment, lost their window sill buttons, and there is a NAC B10M MKII on route 95 that has those buttons replaced with the newer red-and-yellow buttons.
• Especially on some newer SMRT buses and the OAC Nissans, the bus bell has some kind of "anti-abuse" mechanism whereas the bell will not sound after it has been pressed once. While I believe there are good reasons for it to be there (prevents children from playing with it - I was once delayed on a route 859 bus because some child was playing with the bell that had no such mechanism, just before the bus departed the bus stop, and the driver thought someone wanted to alight last minute and had to stop and open the exit doors), it also creates other unintended problems, as mentioned in this thread before - people missing their stops because the driver is too occupied with other things. I almost had this happen on me (TIB 757H on route 171 - I will never forget that one); luckily the bell on that bus didn't have this mechanism, so he could hear the bell when I pressed the bell several times just as he was passing right through my bus stop even though I pressed the bell much earlier, and yet he had the cheek to tell me off for "not pressing the bell early", and told me to "go ahead, go ahead" when I threatened to complain against him (I did it immediately after getting off that bus) for not letting me off despite having pressed the bus bell, which I did (he insists I did not). Imagine if that bus had this mechanism, but usually I will just resort to shouting

. SMRT bus drivers

.
• I know there is one Mercedes MB O405G Hispano MKII that used to be on route 859 whose bell was such a soft beep no one could hear it unless they strained their ears. Worse still, the bus stopping sign has been replaced with those next stop indicators that flash "BUS STOPPING" only every now and then. Many people kept pressing the bell as they thought it was spoilt.
• It seems like that, even though the door closing alarm on the B9TLs seem to be the same as those production Super Olympians, the B9TLs' door closing alarm sounds like a botched-up version of the Super Olympians (it's like "TEAH-TER" rather than the Super Olympians' more pleasant "ding-dong" ) ; anyone know why?
I have many more observations, but I'm too lazy right now to post them here...