There is a preview window on the bottom of the program for playing the video/audio added in. You can click button “Play” to preview your source video/audio and, if you just want to convert a part of video/audio content, you may edit the FLV video with Trim function:

Select the video/audio you have added into the program, go to "Edit" - > "Trim" function, and then you can decide which part can be converted by dragging the left/right slider to set start/end time.

As the picture showing below, drag the left triangle to a position you want it be the start of the converted FLV and drag the right triangle to a position you want it be the end of the converted FLV. Then the video/audio will be trimmed before being converted.

Start time slider - Drag it to set start time.

End time slider - Drag it to set end time.

Navigation slider - Drag it to navigate the source video/audio.

 

You can use keyboard to control the position of slider.

• Left/Right - Go back/forth 0.001s each time.
• Page Up/Page Down - Go back/forth 1s each time.
• Up/Down - Switch to the slider you want to move.