"Usually, a spell-like ability works just like the spell of that name." is what the SRD says.
Paizo says: "A spell-like ability has a casting time of 1 standard action unless noted otherwise in the ability or spell description. In all other ways, a spell-like ability functions just like a spell."
those two things nudge me toward thinking that my dirge bard can fascinate/suggest some undead, but that may be wishful thinking.
Some of the bardic performances are supernatural abilities, some are spell-like abilities.
Two important ones might be Fascinate (supernatural ability) and Suggestion (spell-like ability).
Fascinate (Su): "Fascinate is an enchantment (compulsion), mind-affecting ability."
Suggestion (Sp): "Suggestion is an enchantment (compulsion), mind-affecting, language-dependent ability and relies on audible components."
I'd say RAW says that ability does not equal spell, so secrets of the grave doesn't apply to these. Total bummer.
I'd say they either should re-word the dirge bard description so that undead are not immune to a dirge bard's mind-affecting magical affects because the bard's bardic music is very similar to his spell-casting, or they should add a feat that lets you treat undead as though they were not immune to mind-affecting effects.
