need help on using ffmpeg for audio grab

[Long Wind <longwind2009@gmail.com>]

Another application shows:

Device 00: Cirrus Logic CS4297A rev 4 /dev/dsp
    Port 00: vol
    Port 01: line
    Port 02: mic
    Port 03: cd
    Port 04: line1
    Port 05: phin
    Port 06: phout
    Port 07: video

I enter the command:

ffmpeg -f alsa -ac 1 -i hw:0 t1.mpg

then I test t1.mpg with ffplay,
there's no sound, the line is straight