Sony DR-BT21IK/B Instruction Manual page 19

 Tip
You can control the volume on the headset. (The volume of the iPod is not
changed.) You can also fast-forward or fast-rewind while playing back (page 20).
Notes
 Even if you control the volume on the iPod, sound from the headset will not
change.
 If the transmitter and the headset become out of range and the Bluetooth
connection terminates, repeat the procedure from step 1.
 Playback through a Bluetooth connection may cause noise, sound break, or
playback speed change depending on the location and the usage (page 33).
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