All a speaker-level output is is a giant version of a line-level output . With an appropriate transformer, you can pad that speaker level output back down to something reasonable for a simple opamp like you've linked to, but the transformer required to do that would likely my much more expensive than the amplifier itself. Simply using a ton of resistance won't be great, as you can easily overload the first amplifier's outputs. As for the output of this amplifier itself, taking a look at the schematic we can see that the resistor in parallel with the output is 10ohms. That will change depending on frequency because we also have a capacitor in the same 'branch', but in general we can say that 10ohms is close enough to "speakerland" that this circuit could be used to drive a speaker with no problems, even if the designers did not tell us so.
Getting past that hurdle, take a look at the opamp's datasheet - scroll down the the 'electrical characteristics' section. The maximum output power is going to be limited by the voltage supply rails (simple physics; you can't output more power from a circuit than you're supplying to it). When the supply is a relatively reasonable 9v, peak output power is limited to about 1/3 of a watt. It'll make sound, but probably not as much as you'd like unless you're using an incredibly efficient speaker. How loud does this need to be?