Julian, the SILK codec document says:
SILK can switch in real-time between audio sampling rates of 8, 12, 16 and 24 kHz.
But the payload spec appears to fix the rate: -- the SDP just has a single rate parameter-- using the sample clock directly as the timestamp clock makes it hard to switch between rates.
It appears to me you should be using a 48 kHz timestamp clock to facilitate
switching. Gruesse, Carsten