Have you heard about the concept of fragmentation? If not, have a gaze here: http://www.elegantmedia.com.au/blog/fragmented-app-ecosystem/.
Well, it so happens that the Android operating system of Google is fragmented. There are hundreds of different Android mobile devices all of different shapes, sizes and features. Moreover, Android OS is developed by Google, which doesn't manufacture any of this hardware. So, you can imagine the gap between the software and the hardware. And, no matter how hard mobile device manufacturers try to fill the gap it is impossible for the same OS to dovetailingly (for want of a better term) support all the varied Android devices.
Now, if WordPress were to introduce this audio upload feature, it would have to support all Android mobile devices. It's practically impossible, especially considering how already complicated WordPress CMS is. So, although apps like SoundCloud find it easy because of their limited scope, it's possible but impossible difficult to achieve. And, as Mika said, some older Android OS are extremely limiting and don't support this audio capability, ultimately rendering a broken WordPress experience on these devices.
I hope that answers your question.