It looks like you are passing your data as [channels, length], so you should pass channels_first=True. While the docs says:

channels_first (bool): Downmix across channels dimension.  Default: True

The default seems to be in fact None, which results in dim1 as the default channel dimension:
channels_first = None
ch_dim = int(not channels_first)
print(ch_dim)
> 1

c x n should correspond to [channels, length].
Thanks for reporting this problem! I’ve created an issue here 14.