I don't think there is enough (if any) difference in sound quality in the speakers we use in N scale to make a difference. It's the size that matters (really!), and I buy whatever size I need from whomever makes them with solder pads instead of spring terminals (if I can get them that way). Soberton is the only manufacturer that makes an 8x12mm. Soberton, CUI and PUI all make the other sizes, 16x9, 11x15 and 13x18. I tend to default to CUI for the other sizes, but honestly they are all about the same.
As Tehachipifan notes, the best thing you can do is fit the largest size speaker you can and make sure that you have a suitably-sized SEALED enclosure for it. Minimum enclosure sizes for best sound from these run about 500 cubic mm for the 8x12; 700 for the 9x16 and 11x15, and 1000 for the 13x18. In general, these translate to a box with 6mm (minimum) sides: 8x12x6 = 576; 9x16x6 = 864. 11x15x6=990; 13x18x6 = 1404. By the time you subtract the space taken up by the speaker mechanicals, you come out to about the right number. I actually use 7 - 8mm sides when I can; a slightly larger enclosure than the minimum gives a bit better sound. But you can actually get TOO large, so don't go crazy.
Using these cell-phone type speakers with a suitable enclosure will always blow a factory sound install out of the water. I just replaced a factory sound installation that used two 1" round speakers with a 13x18 CUI in a sealed enclosure, and the sound quality of the 13x18 was so much better the owner of the units couldn't believe it. Manufacturers STILL have not gotten the message that only a sealed enclosure around the speakers gives the best sound. Instead, they continue to mount speakers in or on the frame, using the frame or nothing as an "enclosure" with predictable terrible results. I much prefer to buy engines without sound installed, and I've told my HO-scale friends to do that as well. I can convert them all to sound; use LokSound decoders which are superior (for US diesels, anyway) to anything else on the market and put in a custom speaker for about what the price difference is between a sound and non-sound unit, and my conversions will sound a thousand times better.
John C.
