You should be able to read in your audio files in MATLAB using "audioread". This will give you the audio data (m-by-n matrix, where m is the length of the audio and n is the number of audio channels) and the sample rate. You can refer to the following documentation for more details:
Now for "gmdistribution.fit" you need to input data as an n-by-d matrix, where n is the number of observations and d is the dimension of the data. Assuming that you would like to use the audio data as input to "gmdistribution.fit" and that your audio files have only one channel and are of the same length, you can construct the input using a workflow similar to this:
[data1, Fs] = audioread('audiofile1.wav');
[data2, Fs] = audioread('audiofile2.wav');
input_matrix = [data1 data2];
input_matrix = input_matrix';
obj = gmdistribution.fit(input_matrix, number_of_components);
Note that input to "gmdistribution.fit" should have more rows than columns (i.e. more observations than number of dimensions). Depending on the length of your audio files, their number might not be enough to use a Gaussian mixture model.
Hope this helps!