We are developing an iphone app that needs to process audio data in real time, but we are suffering with performance. The bottlenecks are in audio effects, which are in fact quite simple, but the performance hit is noticeable when several are added.
Most of the audio effects code is written in C.
We think there are two places we can use gpu hardware to speed things up: using openCL for effects and hardware for interpolation/smoothing. We are fairly new to this and don't know where to begin.
You probably mean OpenGL, as OpenCL is only present on the desktop. Yes, you could use OpenGL ES 2.0 programmable shaders for this, if you wanted to perform some very fast parallel processing, but that will be extremely complex to pull off.
You might first want to look at the Accelerate framework, which has hardware-accelerated functions for doing just the kind of tasks needed for audio processing. A great place to start is Apple's WWDC 2010 session 202 - "The Accelerate framework for iPhone OS", along with their "Taking Advantage of the Accelerate Framework" article.
Also, don't dismiss Hans' suggestion that you profile your code first, because your performance bottleneck might be somewhere you don't expect.
You might get better DSP acceleration coding for the ARM NEON SIMD unit. NEON is designed for DSP operations and can pipeline multiple single precision floating point operations per cycle. Whereas getting audio data in and out of GPU memory may be possible, but may not be that fast.
But you might want to profile your code to see if something else is the bottleneck. The iPhone 4 CPU can easily keep up with doing multiple FFT's and IIR filters on a real-time audio stream.