The veneer on your speakers very likely has a factory-applied "varnish" top coat that could be catalyzed conversion type. Even if it is a lesser quality top coat you will not effectively be able to penetrate it with oils. Go to a quality hardware store and inquire about the MinWax line of finishing and wood maintenance products and you may find a good solution. Also purchase a can of Guardsman furniture polish for regular maintenance. Stay away from the stuff sold at the big box hardware stores like Homer Fornsby. And do not ever use Murphy's wax products. Both of these really dry out the wood. If your wood is generally exposed (meaning you do not see a finished top coat) try working a more out of sight area using linseed oil. I sell cabinetry so I am not faking it here. You are welcome to email me if you care to discuss further.
John
John