Right click on the MEDIA in the MEDIA POOL. Hold that while you drag it to the timeline. After you release it will ask if you want video, audio, or both. There are other ways but this one. To delete the video track without deleting audio track, you need to ungroup the event.
Depending on what you wish to do, you could also click the mute button on the audio track that comes with the video. Then insert a new audio track below that one and put whatever you want there. This way the original audio is still available by temporarily clicking the mute button again if you need to check something for timing.
If you put the clip in the trimmer window and press the "tab" key on the keyboard, you can drop the clip without the sound into your timeline at the cursor. Or you can press the tab key twice and place only the audio on the timeline. You must have the trimmer window activated first for this to happen (just put the cursor in the trimmer window).