I have installed sampmlerbox (downloaded it from a dutch website)
I connected a usb midi keyboard and was able to get some sound out of it but when I use an usb sound interface, I have noticeable latency. I measured it and I get 15 to 20ms. When I use the integrated output of the RPI the latency is way shorter and it's not noticeable.
I tried to use an I2S DAC (cjmcu-5102) but didn't manage to get it work.
I don't know how to solve this issue. (I tried two different usb audio interface)
Please see the FAQ "I experience / measure (too) much latency" on the site where you downloaded your samplerbox (I expect it was the "homspace" site).
Please note that both the image and site changed on August 10th, so you may have to refresh your browser and possibly use the updated image.
As for the cjmcu-5102, I understand from its docs you need to configure it on samplerbox as HifiBerry DAC and power via a 5V pin.
I achieved to make it work ...
So Samplerbox doesn't always send signal to the i2s pins, it has to be set up in the configuration files.
The cjmcu-5102 requires the pins FLT, DMP, SCL and FMT to be connected to GND, VCC to 5V, XMT to 3,3V and BCK, DIN and LCK to the corresponding RPI pins