There is a driver file that you will need to install to get sound in win 7. It will be installed in the win 7 guest.This thread
may help as it contains a download link to the necessary file.
Desktop: AMD 3.6ghz X4, Gigabyte MB, 8GB ram, Multiple PCLOS's, 220.127.116.11-pclos3.pae.bfs
Server: Intel 3.4ghz X2, 4GB RAM, SATA, gnome2, *.pae.bfs, Headless, Servers: LAMP, FTP, Samba, Print, PXE, BT
HTPC: Phenom II X4 2.8ghz, 4GB Ram, 1TB SATA, PCLinuxOS-KDE, NVIDIA N220GT HDMI, *.pae.bfs