It has been some time since I messed with this code -- it was a personal project that I haven't been back to for awhile and not at all refined, but I just tested it again and it is working.

//JS//

I'm using drag and drop in the UI so in my drop event I have:

var files = e.originalEvent.dataTransfer.files;
instance._uploadFile(files[0]);

And then:

_uploadFile: function (file) {
	var instance = this;
	var xhr = new XMLHttpRequest();

	if (xhr.upload && file.size <= instance.options.maxFileSize) {

		xhr.upload.addEventListener("progress", function (e) {
			var progress = parseInt(100 - (e.loaded / e.total * 100));
			instance.element.find('.progress').css({ width: progress + '%' }).show();
		}, false);

		xhr.onreadystatechange = function (e) {
			if (xhr.readyState == 4) {
				instance.element.find('.progress').addClass((xhr.status == 200 ? "success" : "failure"));
			}
		};

		var S = $.ServicesFramework(instance.options.moduleId);

		var url = S.getServiceRoot('VoxDynamic.OM') + "/AudioFile/Upload";

		var formData = new FormData();
		formData.append("file", file);

		xhr.open("POST", url, true);                
		xhr.setRequestHeader("X_PORTALID", instance.options.portalId);                
		xhr.send(formData);
	}
}

//AudioFileController.cs//

And for the web service:

public class AudioFileController : DnnApiController
{
	[AllowAnonymous]
	[HttpPost]
	[ValidateAntiForgeryToken]
	public async Task<HttpResponseMessage> Upload()
	{
		var portalId = Convert.ToInt32(Request.Headers.Where(x=> x.Key == "X_PORTALID").Single().Value.First());

		if (!Request.Content.IsMimeMultipartContent())
		{
			throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.UnsupportedMediaType));
		}

		var provider = new FilteredMultipartMemoryStreamProvider(new string[] { "mp3" });

		await Request.Content.ReadAsMultipartAsync(provider);

		if (provider.Contents.Count > 0)
		{
			var content = provider.Contents.Single();
			var stream = await content.ReadAsStreamAsync();
			IFileInfo tempFile;

			if (!FolderManager.Instance.FolderExists(portalId, "Audio"))
			{
				CreateAudioFolder(portalId);
			}

			var audioFolder = FolderManager.Instance.GetFolder(portalId, "Audio");

			if (stream.Length != 0)
			{
				tempFile = FileManager.Instance.AddFile(audioFolder, Guid.NewGuid().ToString() + ".mp3", stream);
				return Request.CreateResponse(HttpStatusCode.OK, tempFile.FileId);
			}
		}

		throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.UnsupportedMediaType, "Count < 1"));

	}	
}

public class FilteredMultipartMemoryStreamProvider : MultipartMemoryStreamProvider
{
	private string[] _extensions;

	public FilteredMultipartMemoryStreamProvider(string[] extensions)
	{
		_extensions = extensions;
	}

	public override System.IO.Stream GetStream(HttpContent parent, System.Net.Http.Headers.HttpContentHeaders headers)
	{
		var filename = headers.ContentDisposition.FileName.Replace("\"", string.Empty);

		if (filename.IndexOf('.') < 0)
			return Stream.Null;

		var extension = filename.Split('.').Last();

		return _extensions.Any(i => i.Equals(extension, StringComparison.InvariantCultureIgnoreCase))
				   ? base.GetStream(parent, headers)
				   : Stream.Null;

	}
}