Acoustical signals in digital form can include additional information, this information is separate from the original signal even though they are both stored in a same file. Methods for storing additional information into the original acoustical signal itself have been investigated. Their bit-rate is at most several hundred bit per second. This paper proposes a new method for digital watermarking with higher bit-rate. Its information hiding and detection method is based on spline interpolation. As a result of listening experiment, bit-rate of above 1 kbps was achieved without audible deterioration.