Want to create bluetooth audio control (volume up/down, play, pause, etc.) for smartphone
Bluetooth supports multiple protocols for multiple purposes. There is low latency low quality monorail bi-directional communication with a little bit of bandwidth for control for Bluetooth phone calls. This is called HFE Bluetooth protocol. If you are interfacing a keyboard or a mouse you would use the Human Interface Device protocol or HID. If you wanted to listen to music and control it's playback you would use the high latency high quality 2 audio sending channels (stereo) no return audio channel with a little bit of bandwidth for controlling playback called Advanced Audio Distribution Profile or A2DP.
In this specific case we are only interested in controlling playback. So we could use the HID Bluetooth protocol. However, the native music players on the platform you are using (Android or iPhone) likely will not be listening to such a device. In order for this to work you will probably have to write your own Android or iPhone music player.
On the other hand, it is likely the native music players will work with A2DP Bluetooth protocol. Unfortunately, it is also suspected that once the A2DP Bluetooth connection is established, the phone's speaker will be shutdown and all audio will be diverted to the Bluetooth connection.
Finally, there is the Audio/Video Remote Control Profile or AVRCP. This protocol is used to control media playback. There are several versions of this protocol so what features are available for a given device / music-application may vary greatly.
I can add something to @st2000's excellent answer (in a better format than our discussion in the comments there).
The original question is vague. Does Bluetooth mean Classic or is Low Energy (BLE) acceptable or preferred. Which smartphones must be supported? Apple and/or Android? And what minimum version must be supported?
The 2.5 years that have passed since this question was asked makes it easier to choose BLE because adoption is so prevalent now.
I've been researching what it would take to implement a media control device as described in the original post. My target smartphone is Android Nougat. My goal is to skip podcast commercials. I have tested using a Bluetooth Classic keyboard. The media control buttons, Play/Pause, skip forward, skip back, etc. seem to do what I need.
I'd prefer to use BLE's support for keyboards and other Human Interface Devices: HID over Gatt Profile (HOGP). The KitKat release notes say that's when HID over GATT support was added, so it's been around for a while.
Having decided on a Bluetooth LE profile, device selection remains. Adafruit Bluefruit LE UART Friend has a HIDKeyboard example. I prefer an MCU with integrated BLE support. While total overkill for this application, I have an ESP32 that I plan to use for this. It has good community support and is cheap. If battery life (or size) becomes a problem, I'll look for a lesser device.
I did it on HC-05 (reflash to RN-42) and use HID profile to send command: Play/pause fwd/rwd vol+/- I can post sample code if needed. Here's the video https://youtu.be/y8PcNbAA6AQ