I have 3 different apps on my Android phone and 1 on an iPad. They all have room for improvement and their effectiveness and features depend on the platform (Android or iOS) and the device. I have Sibley, Audubon, and iBird. Sibley is my favourite, but it's also the most expensive (and therefore the last one I purchased). They all have a good selection of songs and calls. I would start with Audubon. It's a pretty decent app, especially on iOS, and I think it is currently the least expensive. All the Audubon guides go on sale frequently. It was on for 99ยข a couple of weeks ago.
Beware of storage space: they all have huge databases that must be installed.
All the apps fail to various degrees and in various ways to fully exploit the potential of a mobile app versus a printed book, but they're getting there. They are though by far the fastest an cheapest way to get a huge collection of bird recordings.
BB