You want to add pod 'RBVolumeButtons', '~> 0.0' similar to the following to your Podfile

target 'MyApp' do
  pod 'RBVolumeButtons', '~> 0.0'
end

然后在终端内或从 CocoaPods.app 中运行 pod install 命令。

或者为了进行测试运行，运行以下命令

pod try RBVolumeButtons