In your code snippet you are using hybrid controls instead of aura controls.  When you do that the get("v.value") method does not work.  You can use javascript to reference the html element but it might be better to use the ui:inputSelect and ui:inputSelectOption controls.

Check out the docs: [UI_Select][1]


  [1]: https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/ui_select.htm