AAC encoded data can come in many container formats . Like Mp4 ,adts etc., . In any of these are the sampling rates allowed to change inbetween a file ?If so are there any decoders which support it ?
The sampling rate is fixed at encode time.
However for aac-he files a decoder can optionally downsample by 2 if it does not support he. This is because he files are actually encoded at half sample rate and then upsampled by the decoder to the original rate.
Also note that lossy encoders only encode as little as possible, so even if a sample rate was 192KHz, but there was only audio (or a lowpass filter) under 16000 Hz then that would be effectively the same as a sampling rate of 32KHz.