You can use VoiceOver gestures to trim Camera videos and Voice Memo recordings.
Trim a voice memo: On the Voice Memos screen, select the button to the right of the memo you want to trim, then double-tap. Then select Trim Memo and double-tap. Select the beginning or end of the trim tool. Swipe up to drag right, or swipe down to drag left. VoiceOver announces the amount of time the current position will trim from the recording. To complete the trim, select Trim Voice Memo and double-tap.
Trim a video: While viewing a video in Photos, double-tap the screen to display the video controls, then select the beginning or end of the trim tool. Then swipe up to drag to the right, or swipe down to drag to the left. VoiceOver announces the amount of time the current position will trim from the recording. To complete the trim, select Trim and double-tap.