Yes, you can trim your audio clip in the PosterMyWall editor.
When you add an audio clip to your design, the Timeline editor and a right-side panel with audio editing options will appear.
You can trim the audio by dragging the handles on either side of the audio clip in the Timeline editor. Alternatively, click on 'Trim' on the right-side panel and then adjust the length of your audio clip by dragging the audio handles on either side or increasing/decreasing the start and end time values.
Click the blue 'Trim' button when done to save changes.
You can also use the right-side panel to duplicate, replace, adjust the volume and speed of, and apply fade effects to your audio clips.