Can I use any of Sillab MCUs to convert USB...
The F32x and F34x series MCUs have a USB interface, whereas the F34x have more ports, Flash/RAM and are faster.

...into digital audio format ?
I am working on a USB speaker for PC project .
It depends on your application. Generally I'd say yes, it's possible, but since the SiLabs parts have only full-speed capability, it might be hard.
I2S is not possible directly, but it should be possible using e.g. SPI. Currently I'm on the beginning of a project where I use a I2S audio DAC with a SiLabs MCU. I think I'll begin with prototype board on weekend, and coding will begin next week. We can compare our results.
My first approach will be to use the SPI interface for I2S data.

Regards,

Ya, I2S-like signaling is generated using SPI and PCA.
"Enhanced" SPI on SiLabs 'F32x/34x can put data bits continuously, when two bytes are written to SPIDAT, one after another immediately. The second byte is held in the transfer buffer on the SPI, until the first byte shifts out. When single byte transfer finishes, next byte (the third and latter) is fed by SPI interrupt.
In this case, there is a short gap of two SYSCLK cycles from the end of the last byte period, to the start of the next byte period.
PCA Frequency Output mode generates Word Select signaling without any firmware intervention. The start phase of PCA is tuned with the difference between PCA timer (PCA0L) and PCA0CPLn value. These values are set so that the edge of PCA toggle matches to the SPI clock, to place the WS edge at one SCK ahead to the SD word boundary.

SYSCLK is supplied by on-chip "external" OSC (EXOSC) with crystal, which works as the base clock for SPI and PCA (ie. I2S, too)
USB uses on-chip "internal" OSC (IOSC) and clock multiplier, which synchronizes to SOF on USB. These two OSCs work independently.

Tsuneo