Generating a sinusoid sound signal in Java
There's probably good room for performance gain regarding the Math.sin call itself. You could easily create a lookup-table - there's no faster way than just taking the values straight from an array. (The link's benchmark measures a factor ~6 speed difference.)

It may be problematic because it may cause audible distortions, but that would depend on how much memory you're willing to invest in the lookup-table. Also, you could invest a few more cycles by obtaining both array entries addressed by (_tone*i) and then adding them up, both multiplied by 0.x and respectively (1-0.x), effectively fading over between values.