There appear to be inaccuracies in OFP's directional sound techniques. Whether these are caused by OFP itself, by sound cards or by DirectX remains debatable. Some people have these problems with EAX sound support enabled, others with EAX disabled.
A workaround for this issue was implemented in the OFP: Resistance version 1.85 upgrade. Users of this or a later upgrade should not experience this problem with any sound card or driver version. See this BIS FAQ entry.