Merge Clips is greyed out
I shot video on Canon 5D, converted to ProRes and imported into FCP 7. Audio was recorded separately on a JVC GY-HM100U (lens shut, so no video footage just sound) and imported into FCP. I've marked in on video and audio clips to merge them, but the merge clips button is greyed out. The clips are in the browser window. I've tried dragging them into the timeline and merging them there. Merge Clips is still greyed out.
Help please!
Details: Audio- 48KHz, 16-bit
Video- 44.1KHZ
Final Cut Pro 7