Just tap the “Edit” button on the recording management page. (Figure 1)
Then tap the “-” to select a recording (Figure 2.1) , then “Delete” (Figure 2.2)
Tap the “Done” button to finish deleting files. (Figure 2.3)
Warning: The “Delete” operation can not undo. The file will be deleted permanently after you tap it. You can not get the file back unless you have backup it.
To delete all recordings, reference: