The first one shows the battery level if reported.
The second one is showing that a wireless headset or speakers are connected.
Why is a speaker represented by an headphones icon?
That is the default icon for all audio connection.
Why do the icons differ?
The first one is when the devices battery level, when not reported it wont show.
What are their respective meanings?
The first tone is the device is charging. The second one that some speakers or a headset is connected.
Does the absence of a battery indicator mean that the speaker doesn't report battery status?
Yes. To view the battery while the device battery is not displayed, there is a battery widget available when swiping right on the first page of the Home Screen.
Apple recommend answer:
The icon is displayed if the device sends a signal to the iPhone to report the connection and battery status. That is something new for the iPhone. I agree with deggie that the icon is just a standard, they don't have different icons for different devices. It is just reporting the device connected and the battery status of the device.