The headphone jack is a combined 3.5mm audio jack and mini-TOSLINK. The Express will automatically switch from analogue (electrical current over the copper contacts) to digital (red laser over optical) when it detects a mini-TOSLINK cable.
It sounds like the detection circuit is malfunctioning and switching to digital. Try cleaning the port, checking for debris, or gently inserting and removing an audio jack several times.
If it still doesn’t switch back to analogue, you can use speakers that support TOSLINK. If you get speakers that use TOSLINK, you’ll need a Mini-TOSLINK to TOSLINK cable. Otherwise, if the both use mini, then you want mini on both sides.