  The macroscopic friction of particulate materials often weakens as the flow
rate is increased, leading to potentially disastrous intermittent phenomena
including earthquakes and landslides. We theoretically and numerically study
this phenomenon in simple granular materials. We show that velocity-weakening,
corresponding to a non-monotonic behavior in the friction law $\mu(I)$, is
present even if the dynamic and static microscopic friction coefficients are
identical, but disappears for softer particles. We argue that this instability
is induced by endogenous acoustic noise, which tends to make contacts slide,
leading to faster flow and increased noise. We show that soft spots, or
excitable regions in the materials, correspond to rolling contacts that are
about to slide, whose density is described by a nontrivial exponent $\theta_s$.
We build a microscopic theory for the non-monotonicity of $\mu(I)$, which also
predicts the scaling behavior of acoustic noise, the fraction of sliding
contacts $\chi$ and the sliding velocity, in terms of $\theta_s$. Surprisingly,
these quantities have no limit when particles become infinitely hard, as
confirmed numerically. Our analysis rationalizes previously unexplained
observations and makes new experimentally testable predictions.
