the MIDI output bus (where I suppose you're inserting the LSP Multi-Sampler plugin) should have its audio routed (via plugin-box rclick-menu > Audio > ...) to the 26 channel audio output bus named "LSP".
note that on the later you really only need the "output" bus part, though you seem to have created a "duplex" one, which means that you've 26 too many input ports that you won't ever use for nothing. iow. all those LSP/in_1..26 ports are just useless.
maybe not so obvious...
the MIDI output bus (where I suppose you're inserting the LSP Multi-Sampler plugin) should have its audio routed (via plugin-box rclick-menu > Audio > ...) to the 26 channel audio output bus named "LSP".
note that on the later you really only need the "output" bus part, though you seem to have created a "duplex" one, which means that you've 26 too many input ports that you won't ever use for nothing. iow. all those LSP/in_1..26 ports are just useless.
hth.
cheers