Playback hassles

I am new to MuseScore and have installed it on my PC and laptop. On both computers, I've created a melody for soprano and a bass line with the bass guitar. The playback works perfectly on my PC and both parts
> can clearly be heard. Using the laptop, the soprano line only is played back. As it progresses, the notes turn blue on both staves. I've checked the mixer and the bass isn't muted and the volume is at the same level as the soprano's part. Any clues? I'd appreciate any help you can offer here.
(The PC has MuseScore 9.5 and the laptop 9.4. The PC uses Windows 98 and the laptop uses Windows 7)