AWS S3 Get object not working. Getting corrupt file

C# Directly supports writing the stream to a file:

using (var response = await S3Client.GetObjectAsync(request))
using (var responseStream = response.ResponseStream)
using (var outputFileStream = new FileStream(songPath, FileMode.Create))
{
    await responseStream.CopyToAsync(outputFileStream);
}

Ok. Fiddled with your code a bit. And got NoSupported errors on responseStream.Length.

I changed to code to using response.ContentLength and it started working for me.

This is the code I ended up with:

async Task<byte[]> GetZip(string pFile)
{
    try
    {
        Console.WriteLine("KEY: " + pFile);
        Amazon.S3.Model.GetObjectRequest request = new Amazon.S3.Model.GetObjectRequest
        {
            BucketName = S3Bucket,
            Key = pFile
        };
        using (Amazon.S3.Model.GetObjectResponse response = await S3Client.GetObjectAsync(request))
        using (Stream responseStream = response.ResponseStream)
        {
            Console.WriteLine("Response stream");
            if (responseStream != null)
            {
                byte[] buffer = new byte[(int)response.ContentLength];
                int result = await responseStream.ReadAsync(buffer, 0, (int)response.ContentLength);
                Console.WriteLine("Stream result: " + result);
                File.WriteAllBytes(songPath, buffer);
                Console.WriteLine("Readed all bytes: " + buffer.Length + " - " + ((int)response.ContentLength));
                return buffer;
            }
            else
            {
                Console.WriteLine("Response is null");
            }
        }
    }
    catch (Amazon.S3.AmazonS3Exception e)
    {
        // If bucket or object does not exist
        Console.WriteLine("Error encountered ***. Message:"+ e.Message + " when reading object");
    }
    catch (Exception e)
    {
        Console.WriteLine("Unknown encountered on server. Message:"+ e.Message + " when reading object");
    }
    return new byte[0];
}

GetZip(pFile).Wait();

Is the file truly corrupt or has it been unzipped and you didn't expect it to be? From memory the Content Encoding metadata on the object in S3 will influence whether a zip file is transparently unzipped on the fly.