Voices from upstairs while I work

I work from home and am located in the basement. I've been alone in the house for most of the day. Around 30 minutes ago I kept hearing a young female voice upstairs. It was very muffled and I couldn't tell what she was saying. It was also very intermittent. I would hear the voice and several seconds would go by, and I wouldn't hear anything. So I would go back to my work, only to hear it again a minute or so later!

I finally got tired of this and walked softly over to to the landing of the basement steps. I put my voice recorder on the landing, and turned it on. My plan was that any noises from upstairs should echo down the stairwell and be captured by the sound recorder.

I went back to my desk and continued working for around 5 minutes, after which I got up to retrieve the recorder. During that time I did hear the voice several times and was excited to listen to it on the recorder.

Alas, the recorder picked up no voices. I was more than a little disappointed because I know I heard the voice off and on for a period of about 30 minutes or more.